Browser compatibility

Alternate rendition of Fyrd's Can I use page with Wikimedia browser usage statistics.

featureAnd.iOSChromeEdgeSafariSSFF%
12515161717.5109123124125126125161717.525126127
featureAnd.iOSChromeEdgeSafariSSFF%
webkitmoz
css2CSS min/max-width/height

Method of setting a minimum or maximum width or height to an element.

Browser-specific notes:
#1: IE7 does not support inherit as a value on any of these properties.
#2: IE8 has some bugs with max-width/height combined with overflow: auto/scroll.

Resources: JS library with support, WebPlatform Docs, CSS Basics post.
rec2.13.2493.14299
css2CSS 2.1 selectors

Basic CSS selectors including: * (universal selector), > (child selector), :first-child, :link, :visited, :active, :hover, :focus, :lang(), + (adjacent sibling selector), [attr], [attr="val"], [attr~="val"], [attr|="bar"], .foo (class selector), #foo (id selector).

Support for :visited styling varies across browsers due to security concerns.

Resources: Detailed support information, Examples of advanced selectors, Selectivizr: Polyfill for IE6-8, WebPlatform Docs.
rec2.13.2473.14299
pngPNG alpha transparency

Semi-transparent areas in PNG files.

IE6 does support full transparency in 8-bit PNGs, which can sometimes be an alternative to 24-bit PNGs.

Resources: Wikipedia.
rec2.13.2473.14299
css2CSS inline-block

Method of displaying an element as a block while flowing it with text.

Browser-specific notes:
#1: Only supported in IE6 and IE7 on elements with a display of "inline" by default. Alternative properties are available to provide complete cross-browser support.

Resources: Blog post w/info, Info on cross browser support, WebPlatform Docs.
rec2.13.2483.14399
css3CSS3 Opacity

Method of setting the transparency level of an element.

Transparency for elements in IE8 and older can be achieved using the proprietary "filter" property and does not work well with PNG images using alpha transparency.

Resources: WebPlatform Docs.
rec2.13.2493.14299
css3CSS first-line pseudo-element

Allows styling specifically for the first line of text using the ::first-line pseudo-element. Note that only a limited set of properties can be applied.

Browser-specific notes:
#1: IE8 only supports the single-colon CSS 2.1 syntax (i.e. :first-line). It does not support the double-colon CSS3 syntax (i.e. ::first-line).

Resources: MDN Web Docs - ::first-line, CSS tricks article.
Parent feature: CSS3 selectors.
rec2.13.2493.14299
css2CSS Counters

Method of controlling number values in generated content, using the counter-reset and counter-increment properties.

Resources: Tutorial and information, MDN Web Docs - CSS Counters, WebPlatform Docs.
rec2.13.2483.14299
css2CSS Table display

Method of displaying elements as tables, rows, and cells. Includes support for all display: table-* properties as well as display: inline-table.

Browser-specific notes:
#1: Firefox 2 does not support inline-table.

Resources: Blog post on usage.
rec2.13.2483.14399
css2CSS Generated content for pseudo-elements

Method of displaying text or images before or after the given element's contents using the ::before and ::after pseudo-elements. All browsers with support also support the attr() notation in the content property.

For content to appear in pseudo-elements, the content property must be set (but may be an empty string).

Browser-specific notes:
#1: IE8 only supports the single-colon CSS 2.1 syntax (i.e. :pseudo-class). It does not support the double-colon CSS3 syntax (i.e. ::pseudo-element).

Resources: Guide on usage, Dev.Opera article, WebPlatform Docs.
rec2.13.2493.14299
domDocument Object Model Range

A contiguous range of content in a Document, DocumentFragment or Attr.

See MDN for feature support details.

Resources: MDN Web Docs - Range, QuirksMode, "Rangy" Range library with old IE support.
ls2.13.2493.14299
domEventTarget.addEventListener()

The modern standard API for adding DOM event handlers. Introduced in the DOM Level 2 Events spec. Also implies support for removeEventListener, the capture phase of DOM event dispatch, as well as the stopPropagation() and preventDefault() event methods.

Browser-specific notes:
#1: IE<=8 instead only supports the proprietary .attachEvent() method. It also does not support the capture phase of DOM event dispatch; it only supports event bubbling.
#2: The useCapture parameter is non-optional and must be provided. Future versions made it optional, with a default value of false.

Resources: MDN Web Docs - addEventListener, Financial Times IE8 polyfill, WebReflection ie8 polyfill.
ls2.13.2493.14799
domDOMContentLoaded

JavaScript event that fires when the DOM is loaded, but before all page assets are loaded (CSS, images, etc.).

Resources: MDN Web Docs - DOMContentLoaded.
ls2.13.2493.14299
domnaturalWidth & naturalHeight image properties

Properties defining the intrinsic width and height of the image, rather than the displayed width & height.

Resources: Blog post on support in IE, gist on getting natural width & height in older IE.
ls2.13.2493.14299
otherXHTML served as application/xhtml+xml

A strict form of HTML, and allows embedding of other XML languages.

The XHTML syntax is very close to HTML, and thus is almost always (incorrectly) served as text/html on the web.

Resources: Wikipedia, Information on XHTML5.
ls2.13.2493.14299
domgetElementsByClassName

Method of accessing DOM elements by class name.

Resources: Test page, getElementsByClassName on MDN.
ls2.13.2493.14399
domquerySelector/querySelectorAll

Method of accessing DOM elements using CSS selectors.

Browser-specific notes:
#1: Partial support in IE8 is due to being limited to CSS 2.1 selectors and a small subset of CSS 3 selectors (see notes there). Additionally, it will have trouble with selectors including unrecognized tags (for example HTML5 ones).

Resources: MDN Web Docs - querySelector, MDN Web Docs - querySelectorAll, WebPlatform Docs.
ls2.13.2493.143.599
css3CSS3 Text-overflow

Append ellipsis when text overflows its containing element.

Resources: jQuery polyfill for Firefox, MDN Web Docs - text-overflow, has.js test, WebPlatform Docs.
rec2.13.2463.14799
css3CSS3 selectors

Advanced element selection using selectors including: [foo^="bar"], [foo$="bar"], [foo*="bar"], :root, :nth-child(),  :nth-last-child(), :nth-of-type(), :nth-last-of-type(), :last-child, :first-of-type, :last-of-type, :only-child, :only-of-type, :empty, :target, :enabled, :disabled, :checked, :not(), ~ (general sibling).

Browser-specific notes:
#1: IE7 and IE8 support only these CSS3 selectors: General siblings (element1~element2) and Attribute selectors [attr^=val], [attr$=val], and [attr*=val].

Resources: Detailed support information, Automated CSS3 selector test, Selectivizr: Polyfill for IE6-8, WebPlatform Docs.
rec2.13.2493.243.599
css3CSS3 Colors

Method of describing colors using Hue, Saturation and Lightness (hsl()) rather than just RGB, as well as allowing alpha-transparency with rgba() and hsla().

Resources: Dev.Opera article, WebPlatform Docs.
rec2.13.2493.14399
css3CSS3 Media Queries

Method of applying styles based on media information. Includes things like page and device dimensions.

Browser-specific notes:
#1: Does not support nested media queries.
#2: Partial support refers to only acknowledging different media rules on page reload.

Resources: IE demo page with information, Media Queries tutorial, Polyfill for IE, WebPlatform Docs, Practical Guide to Media Queries.
rec4.4726126.143.599
canvasCanvas (basic support)

Method of generating fast, dynamic graphics using JavaScript.

For screen readers, IE, Chrome & Firefox support the accessible canvas element sub-DOM.
Firefox & Chrome also support the drawfocus ring.

Browser-specific notes:
#1: Does not support toDataURL().
#2: Opera Mini supports the canvas element, but is unable to play animations or run other more complex applications.

Resources: Tutorial by Mozilla, Animation kit, Another tutorial, Implementation for Internet Explorer, has.js test, Canvas Tutorial & Cheat Sheet, MDN Web Docs - Canvas API.
ls33.249443.699
jsWeb Storage - name/value pairs

Method of storing data locally like cookies, but for larger amounts of data (sessionStorage and localStorage, used to fall under HTML5).

Resources: MDN Web Docs - Web Storage, Support library, Simple demo, has.js test, WebPlatform Docs.
ls2.13.248443.599
css3CSS3 Text-shadow

Method of applying one or more shadow or blur effects to text.

Opera Mini ignores the blur-radius set, so no blur effect is visible. Text-shadow behavior can be somewhat emulated in older IE versions using the non-standard "dropshadow" or "glow" filters.

Browser-specific notes:
#1: IE 10+ supports a fourth length value for the shadow's "spread". This is not (yet) part of the specification.
#2: Partial support in Safari 3.* refers to not supporting multiple shadows.

Resources: Mozilla hacks article, Live editor, WebPlatform Docs.
cr2.13.2479443.599
css3CSS3 Multiple backgrounds

Method of using multiple images as a background.

Resources: Demo & information page, WebPlatform Docs.
cr2.13.2493.143.699
jsBase64 encoding and decoding

Utility functions for encoding and decoding strings to and from base 64: window.atob() and window.btoa().

Resources: MDN Web Docs - btoa(), MDN Web Docs - atob(), Polyfill.
ls2.13.24103.14299
canvasText API for Canvas

Method of displaying text on Canvas elements.

Resources: Examples by Mozilla, Support library, has.js test, WebPlatform Docs.
Parent feature: Canvas (basic support).
ls2.13.249443.599
jsGeolocation

Method of informing a website of the user's geographical location.

Browser-specific notes:
#1: Only works on secure (https) servers.

Resources: has.js test, WebPlatform Docs, Geolocation API on MDN.
rec12510507910.155599
css3CSS3 Box-sizing

Method of specifying whether or not an element's borders and padding should be included in size units.

Firefox versions before 57 also supported the padding-box value for box-sizing, though this value was been removed from the specification and later versions of the browser.

Resources: MDN Web Docs - CSS box-sizing, Blog post, Polyfill for IE, CSS Tricks, WebPlatform Docs.
wd451085.142999
html5Email, telephone & URL input types

Text input fields intended for email addresses, telephone numbers or URLs. Particularly useful in combination with form validation.

Browsers without support for these types will fall back to using the "text" type.

Browser-specific notes:
#1: Does not provide an email-specific keyboard for email addresses.

Resources: Article on usage.
ls33.251054499
html5input placeholder attribute

Method of setting placeholder text for text-like input fields, to suggest the expected inserted information.

Browser-specific notes:
#1: Partial support refers to lacking placeholder support on textarea elements.

Resources: Article on usage, Polyfill, has.js test, WebPlatform Docs, Issue 24626: Placeholder text for an input type=.
ls4.23.241054499
css3CSS3 Border-radius (rounded corners)

Method of making the border corners round. Covers support for the shorthand border-radius as well as the long-hand properties (e.g. border-top-left-radius).

Browser-specific notes:
#1: Safari 6.1 and earlier did not apply border-radius correctly to image borders: https://stackoverflow.com/q/17202128.
#2: Dotted and dashed rounded border corners are rendered as solid in Firefox. see bug.

Resources: Border-radius CSS Generator, Detailed compliance table, Polyfill which includes border-radius, WebPlatform Docs, MDN Web Docs - CSS border-radius.
cr2.2459745099
html5Video element

Method of playing videos on webpages (without requiring a plug-in). Includes support for the following media properties: currentSrc, currentTime, paused, playbackRate, buffered, duration, played, seekable, ended, autoplay, loop, controls, volume & muted.

Different browsers have support for different video formats, see sub-features for details.

Browser-specific notes:
#1: The Android browser (before 2.3) requires specific handling to run the video element.
#2: Old Firefox versions were missing support for some properties: loop was added in v11, played in v15, playbackRate in v20.
#3: Ignores the autoplay attribute by default, though autoplay behavior can be enabled by users.

Resources: Detailed article on video/audio elements, WebM format information, Video for Everybody, Video on the Web - includes info on Android support, has.js test, WebPlatform Docs.
ls2.311491142099
jsBasic console logging functions

Method of outputting data to the browser's console, intended for development purposes.

The basic functions that this information refers to include console.log, console.info, console.warn, console.error.

Browser-specific notes:
#1: Only supports console functions when developer tools are open, otherwise the console object is undefined and any calls will throw errors.
#2: Allows console functions to be used without throwing errors, but does not appear to output the data anywhere.
#3: Log output on iOS 6+ Safari can only be seen by connecting to a Mac and using the Safari debugger.
#4: Log output on older Android browsers can be retrieved via Android's logcat command or using Chrome Developer Tools in Android 4.4+/Chrome for Android see details.
#5: Log output on Firefox for Android can be accessed using WebIDE.
#6: See this article for details on how to see console logging in Opera Mini.

Resources: MDN Web Docs - Console, Chrome console reference, Edge/Internet Explorer console reference.
ls2.164103.14499
css3CSS3 2D Transforms

Method of transforming an element including rotating, scaling, etc. Includes support for transform as well as transform-origin properties.

The scale transform can be emulated in IE < 9 using Microsoft's "zoom" extension, others are (not easily) possible using the MS Matrix filter.

Browser-specific notes:
#1: Does not support CSS transforms on SVG elements (transform attribute can be used instead).

Resources: Live editor, MDN Web Docs - CSS transform, Workaround script for IE, Converter for IE, has.js test, WebPlatform Docs, Microsoft Edge Platform Status (SVG).
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css3CSS pointer-events (for HTML)

This CSS property, when set to "none" allows elements to not receive hover/click events, instead the event will occur on anything behind it.

Already part of the SVG specification, and all SVG-supporting browsers appear to support the property on SVG elements.

Resources: Article & tutorial, has.js test, Polyfill.
unoff2.13.2411443.699
jsconsole.time and console.timeEnd

Functions for measuring performance.

console.time() starts a timer you can use to track how long an operation takes. You give each timer a unique name, and may have up to 10,000 timers running on a given page. When you call console.timeEnd() with the same name, the browser will output the time, in milliseconds, that elapsed since the timer was started. These functions are not always available in workers. For example, in Firefox, they are available from version 38. More on using the console on mobile devices, see here.

Resources: MDN Web Docs - Console.time.
Parent feature: Basic console logging functions.
ls2.13.2411441099
cssWindow.devicePixelRatio

Read-only property that returns the ratio of the (vertical) size of one physical pixel on the current display device to the size of one CSS pixel.

As the page is zoomed in the number of device pixels that one CSS pixel covers increases, and therefore the value of devicePixelRatio will also increase.

Resources: MDN Web Docs - devicePixelRatio.
wd2.13.24113.141899
domElement.insertAdjacentElement() & Element.insertAdjacentText()

Methods for inserting an element or text before or after a given element, or appending or prepending an element or text to a given element's list of children.

Resources: WHATWG DOM Specification for Element.insertAdjacentText(), MDN Web Docs - Element.insertAdjacentElement(), MDN Web Docs - Element.insertAdjacentText(), JS Bin testcase.
ls2.33.2463.144899
css3@font-face Web fonts

Method of displaying fonts downloaded from websites.

Not supported by IE Mobile 9 and below.

Browser-specific notes:
#1: Partial support refers to only supporting EOT fonts.
#2: Partial support refers to only supporting SVG fonts.

Resources: Wikipedia, WebPlatform Docs.
rec44.2493.243.599
otherData URIs

Method of embedding images and other files in webpages as a string of text, generally using base64 encoding.

Browser-specific notes:
#1: Support is limited to images and linked resources like CSS files, not HTML or JS files. Max URI length is 32KB.
#2: Support is limited to images and linked resources like CSS or JS, not HTML files. Maximum size limit is 4GB.
#3: SVGs with XML declarations are not displayed when used in data-urls.

Resources: Information page, Wikipedia, Data URL converter, Information on security issues.
other2.13.24793.14299
jsCross-document messaging

Method of sending information from a page on one domain to a page on a different one (using postMessage).

Browser-specific notes:
#1: Partial support refers to only working in frames/iframes (not other tabs/windows). Also, objects cannot be sent using postMessage.
#2: Partial support refers to limitations in certain conditions.

Resources: MDN Web Docs - window.postMessage, Simple demo, has.js test, WebPlatform Docs.
ls2.13.241244399
svgSVG (basic support)

Method of displaying basic Vector Graphics features using the embed or object elements. Refers to the SVG 1.1 spec.

Browser-specific notes:
#1: Partial support in Android 3 & 4 refers to not supporting masking.
#2: Partial support in IE9-11 refers to not supporting animations.
#3: IE9-11 & Edge don't properly scale SVG files. Adding height, width, viewBox, and CSS rules seems to be the best workaround.

Resources: Wikipedia, A List Apart article, SVG showcase site, Web-based SVG editor, has.js test.
cr4.43.24793.24399
otherMPEG-4/H.264 video format

Commonly used video compression format.

Firefox supports H.264 on Windows 7 and later since version 21. Firefox supports H.264 on Linux since version 26 if the appropriate system libraries are installed.

Partial support for older Firefox versions refers to the lack of support in OS X & some non-Android Linux platforms.

Browser-specific notes:
#1: The Android 2.3 browser requires specific handling to play videos.

Resources: Wikipedia article, Firefox extension allowing support in Win7.
Parent feature: Video element.
other4.43.2493.243599
css3CSS3 Box-shadow

Method of displaying an inner or outer shadow effect to elements.

Can be partially emulated in older IE versions using the non-standard "shadow" filter.

Browser-specific notes:
#1: Partial support in Safari, iOS Safari and Android Browser refers to missing "inset", blur radius value, and multiple shadow support.

Resources: MDN Web Docs - box-shadow, Live editor, Demo of various effects, WebPlatform Docs.
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css3CSS outline properties

The CSS outline properties draw a border around an element that does not affect layout, making it ideal for highlighting. This covers the outline shorthand, as well as outline-width, outline-style, outline-color and outline-offset.

Browser-specific notes:
#1: Also supports the value of invert for outline-color. (support of this value is optional for browsers).
#2: Does not support outline-offset.

Resources: CSS Basic User Interface Module Level 3, MDN Web Docs - CSS outline.
cr2.13.24153.14299
html5dataset & data-* attributes

Method of applying and accessing custom data to elements.

Partial support refers to being able to use data-* attributes and access them using getAttribute.

"Supported" refers to accessing the values using the dataset property. Current spec only refers to support on HTML elements, only some browsers also have support for SVG/MathML elements.

Browser-specific notes:
#1: While the HTML spec doesn't require it, these browsers also support dataset and data-* attributes on SVG elements, in compliance with current plans for SVG2.

Resources: HTML5 Doctor article, Demo using dataset, has.js test, WebPlatform Docs, MDN Web Docs - dataset, MDN Guide - Using data-* attributes.
ls3555175.145199
css3getComputedStyle

API to get the current computed CSS styles applied to an element. This may be the current value applied by an animation or as set by a stylesheet.

Browser-specific notes:
#1: Partial support refers to requiring the second parameter to be included.
#2: Partial support refers to not supporting getComputedStyle on pseudo-elements.

Resources: MDN Web Docs - getComputedStyle, Demo, Polyfill for IE.
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cssbackground-position-x & background-position-y

CSS longhand properties to define x or y positions separately.

A workaround for the lack of support in Firefox 31 - Firefox 48 is to use CSS variables. See this Stack Overflow answer for an example.

Resources: Firefox implementation bug, Blog post on background-position-x & y properties, MDN Web Docs - background-position-x, MDN Web Docs - background-position-y.
unoff2.13.245.53.144999
css3CSS3 Background-image options

New properties to affect background images, including background-clip, background-origin and background-size.

Firefox, Chrome and Safari support the unofficial -webkit-background-clip: text (only with prefix). Safari does not support -webkit-background-clip: text; for <button> elements. But you can put <span> inside <button> to get the same result.

Browser-specific notes:
#1: Partial support in Opera Mini refers to not supporting background sizing or background attachments. However Opera Mini 7.5 supports background sizing (including cover and contain values).
#2: Partial support in Safari 6 refers to not supporting background sizing offset from edges syntax.
#3: Does not support background-size values in the background shorthand.

Resources: Detailed compatibility tables and demos, Polyfill for IE7-8, MDN Web Docs - background-image.
cr4.4715974499
css3CSS3 Transitions

Simple method of animating certain properties of an element, with ability to define property, duration, delay and timing function.

Support listed is for transition properties as well as the transitionend event.

Browser-specific notes:
#1: Does not support the steps(), step-start & step-end timing functions.

Resources: Article on usage, Examples on timing functions, WebPlatform Docs.
wd4.4726106.141699
css3SVG in CSS backgrounds

Method of using SVG images as CSS backgrounds.

Browser-specific notes:
#1: Partial support in iOS Safari and older Safari versions refers to failing to support tiling or the background-position property.
#2: Partial support in older Firefox and Opera Mini/Mobile refers to SVG images being blurry when scaled.
#3: Partial support in Edge 15 and older refers to a lack of support for SVG data URIs. see bug.

Resources: Tutorial for advanced effects.
cr34.2516542499
jsCross-Origin Resource Sharing

Method of performing XMLHttpRequests across domains.

Browser-specific notes:
#1: Does not support CORS for images in <canvas>.
#2: Supported somewhat in IE8 and IE9 using the XDomainRequest object (but has limitations).
#3: Does not support CORS for <video> in <canvas>: https://bugs.webkit.org/show_bug.cgi?id=135379.
#4: Does not support CORS for resources which redirect: https://bugzilla.mozilla.org/show_bug.cgi?id=1346749.

Resources: Mozilla Hacks blog post, Alternative implementation by IE8, DOM access using CORS, has.js test, MDN Web Docs - Access control CORS.
ls4.461311647199
svgSVG in HTML img element

Method of displaying SVG images in HTML using <img>.

Browser-specific notes:
#1: Partial support refers to not supporting embedded images (data URIs) inside the SVG.

Resources: Blog with SVGs and images.
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css3CSS Animation

Complex method of animating certain properties of an element.

Browser-specific notes:
#1: Partial support in Android browser refers to buggy behavior in different scenarios.
#2: Does not support the steps(), step-start & step-end timing functions.

Resources: Blog post on usage, WebPlatform Docs.
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jsECMAScript 5

Full support for the ECMAScript 5 specification. Features include Function.prototype.bind, Array methods like indexOf, forEach, map & filter, Object methods like defineProperty, create & keys, the trim method on Strings and many more.

As the specification includes many JavaScript features, un-numbered partial support varies widely and is shown in detail on the ECMAScript 5 compatibility tables by Kangax.

Browser-specific notes:
#1: Does not support parseInt() ignoring leading zeros.
#2: Does not support Strict mode.
#3: Does not support zero-width chars in identifiers & Immutable undefined.
#4: IE8 has virtually no ES5 support, but does support Object.defineProperty, Object.getOwnPropertyDescriptor, JSON parsing & Property access on strings.

Resources: Detailed compatibility tables & tests, Overview of objects & properties, ES5 polyfill, Polyfill for all possible ES5 features is available in the core-js library.
other4.462310642199
domDOM Parsing and Serialization

Various DOM parsing and serializing functions, specifically DOMParser, XMLSerializer, innerHTML, outerHTML and insertAdjacentHTML.

Browser-specific notes:
#1: Partial support refers to lacking support for parseFromString on the DOMParser.
#2: Partial support refers to supporting only innerHTML.
#3: Partial support refers to supporting only innerHTML and insertAdjacentHTML.

Resources: MDN Web Docs - XMLSerializer, Comparing Document Position by John Resig.
cr4.4831107.141299
cssCSS position:fixed

Method of keeping an element in a fixed location regardless of scroll position.

Browser-specific notes:
#1: Partial support in older iOS Safari refers to buggy behavior.
#2: Only works in Android 2.1 thru 2.3 by using the following meta tag: <meta name="viewport" content="width=device-width, user-scalable=no">.
#3: Android 4.0-4.3 ignore transforms and margin:auto on position:fixed elements.
#4: in Safari 9.1, having a position:fixed-element inside an animated element, may cause the position:fixed-element to not appear.

Resources: Article on mobile support, position: fixed on MDN.
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cssCSS namespaces

Using the @namespace at-rule, elements of other namespaces (e.g. SVG) can be targeted using the pipe (|) selector.

Resources: MDN Web Docs - CSS @namespace.
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cssCSS currentColor value

A CSS value that will apply the existing color value to other properties like background-color, etc.

Resources: MDN Web Docs - CSS currentColor, CSS Tricks article.
rec2.144944299
domEventTarget.dispatchEvent

Method to programmatically trigger a DOM event.

Browser-specific notes:
#1: Supports Microsoft's proprietary EventTarget.fireEvent() method.

Resources: MDN Web Docs - dispatchEvent, Financial Times IE8 polyfill, WebReflection ie8 polyfill.
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domNode.textContent

DOM Node property representing the text content of a node and its descendants.

Node.textContent is somewhat similar to, but has important differences from, HTMLElement.innerText.

Resources: MDN Web Docs - Node.textContent.
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securityServer Name Indication

An extension to the TLS computer networking protocol by which a client indicates which hostname it is attempting to connect to at the start of the handshaking process.

Browser-specific notes:
#1: Only supported on Windows Vista or above (not Windows XP).

Resources: Wikipedia.
other34693.14299
jsJSON parsing

Method of converting JavaScript objects to JSON strings and JSON back to objects using JSON.stringify() and JSON.parse().

Browser-specific notes:
#1: Requires document to be in IE8+ standards mode to work in IE8.

Resources: MDN Web Docs - Working with JSON, JSON in JS (includes script w/support), has.js test, WebPlatform Docs, JSON explainer.
Parent feature: ECMAScript 5.
other2.1449443.599
html5Inline SVG in HTML5

Method of using SVG tags directly in HTML documents. Requires HTML5 parser.

Browser-specific notes:
#1: Does not support CSS transforms on SVG elements (transform attribute can be used instead).

Resources: Mozilla Hacks blog post, Test suite.
ls4.4971794499
html5contenteditable attribute (basic support)

Method of making any HTML element editable.

This support only refers to very basic editing capability, implementations vary significantly on how certain elements can be edited.

Resources: WHATWG blog post, Blog post on usage problems, WebPlatform Docs, MDN Web Docs - contentEditable attribute.
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html5Selection controls for input & textarea

Controls for setting and getting text selection via setSelectionRange() and the selectionStart & selectionEnd properties.

Resources: MDN article on setSelectionRange.
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html5Hashchange event

Event triggered in JavaScript when the URL's hash has changed (for example: page.html#foo to page.html#bar).

Resources: MDN Web Docs - onhashchange, Simple demo, Polyfill, WebPlatform Docs.
ls2.2458543.699
jslocaleCompare()

The localeCompare() method returns a number indicating whether a reference string comes before or after or is the same as the given string in sort order.

Browser-specific notes:
#1: Only supports basic support without locale & options parameters.

Resources: MDN article.
other4.41024111052999
jsElement.getBoundingClientRect()

Method to get the size and position of an element's bounding box, relative to the viewport.

Browser-specific notes:
#1: The returned object lacks width and height properties.
#2: Returns incorrect values for elements which have had CSS transforms applied to them.
#3: The returned object cannot have new properties added to it; it's not extensible.
#4: Existing properties of the returned object are immutable.
#5: The returned object lacks x & y values, though existing top & left values can be used instead.

Resources: MDN Web Docs - getBoundingClientRect, Microsoft Developer Network.
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domdocument.evaluate & XPath

Allow nodes in an XML/HTML document to be traversed using XPath expressions.

Resources: XPath in Javascript: Introduction, MDN Web Docs - XPath introduction, Edge team article on implementation, DOM XPath - WHATWG Wiki.
unoff2.13.24123.14399
jsElement.insertAdjacentHTML()

Inserts a string of HTML into a specified position in the DOM relative to the given element.

Browser-specific notes:
#1: Throws an "Invalid target element for this operation." error when called on a table, tbody, thead, or tr element..

Resources: MDN Web Docs - insertAdjacentHTML, Polyfill.
cr2.3441044899
css3CSS3 3D Transforms

Method of transforming an element in the third dimension using the transform property. Includes support for the perspective property to set the perspective in z-space and the backface-visibility property to toggle display of the reverse side of a 3D-transformed element.

Browser-specific notes:
#1: Partial support in IE refers to not supporting the transform-style: preserve-3d property. This prevents nesting 3D transformed elements.
#2: Safari 9-15.3 still require a prefix for the related backface-visibility property.

Resources: Multi-browser demo, Mozilla hacks article, 3D CSS Tester, has.js test, WebPlatform Docs, Intro to CSS 3D transforms.
wd125915.4361215.441699
dommaxlength attribute for input and textarea elements

Declares an upper bound on the number of characters the user can input. Normally the UI ignores attempts by the user to type in additional characters beyond this limit.

Browser-specific notes:
#1: <textarea>'s' UI does not prevent the user from typing additional characters beyond the maxlength limit.
#2: Does not support the HTMLTextAreaElement.maxLength DOM property.
#3: Does not support ValidityState.tooLong. In some cases, this is because .validity is not supported in the first place.
#4: Does not support ValidityState.tooLong correctly in the (unlikely) case of the value being initially set too long, then changed by the user to a still incorrect state. See Firefox bug and MS Edge bug.
#5: <input>'s UI does not prevent the user from typing additional characters beyond the maxlength limit between two existing characters of the string.
#6: Allows text beyond maxlength to be entered at first, but removes all characters past the maxlength when focus is lost.

Resources: MDN Web Docs - attribute maxlength.
Parent feature: Form validation.
ls12594795.145199
domdocument.head

Convenience property for accessing the <head> element.

Resources: MDN Web Docs - head.
ls2.34495.14499
jsWeb Workers

Method of running scripts in the background, isolated from the web page.

Resources: MDN Web Docs - Using Web Workers, Web Worker demo, Polyfill for IE (single threaded), Tutorial.
ls4.45410443.599
css::first-letter CSS pseudo-element selector

CSS pseudo-element that allows styling only the first "letter" of text within an element. Useful for implementing initial caps or drop caps styling.

The spec says that both letters of digraphs which are always capitalized together (such as "IJ" in Dutch) should be matched by ::first-letter, but no browser has ever implemented this.

Browser-specific notes:
#1: Excludes punctuation immediately after the first letter from the match. (The spec says it should be included in the match.).
#2: Acts like the first character is always a letter even when it's not. For example, given "!,X;", "!," is matched instead of the entire string.
#3: Only recognizes the deprecated :first-letter pseudo-class, not the ::first-letter pseudo-element.
#4: Only matches the very first character. The spec says that surrounding punctuation should also match.

Resources: MDN Web Docs - :first-letter.
rec35995.143.599
css3rem (root em) units

Type of unit similar to em, but relative only to the root element, not any parent element. Thus compounding does not occur as it does with em units.

Browser-specific notes:
#1: IE 9 & IE 10 do not support rem units when used in the font shorthand property (the entire declaration is ignored) or when used on pseudo elements.
#2: iOS Safari 5.0-5.1 support rem but not in combination with media queries.

Resources: Article on usage, REM Polyfill.
cr2.16411543.699
svgSVG filters

Method of using Photoshop-like effects on SVG objects including blurring and color manipulation.

Resources: SVG filter demos, WebPlatform Docs, SVG Filter effects.
rec4.4681064399
domdocument.elementFromPoint()

Given coordinates for a point relative to the viewport, returns the element that a click event would be dispatched at if the user were to click the point (in other words, the element that hit-testing would find).

Resources: MDN Web Docs - elementFromPoint.
wd2.3415654399
jsDocument.execCommand()

Allows running commands to manipulate the contents of an editable region in a document switched to designMode.

To determine what commands are supported, see Document.queryCommandSupported().

Resources: MDN Web Docs - execCommand, execCommand and queryCommandSupported demo.
unoff4.1745.564999
css3CSS3 word-break

Property to prevent or allow words to be broken over multiple lines between letters.

Partial support refers to supporting the break-all value, but not the keep-all value.

Chrome, Safari and other WebKit/Blink browsers also support the unofficial break-word value which is treated like word-wrap: break-word.

Resources: MDN Web Docs - CSS word-break, WebPlatform Docs.
cr1259445.5941599
otherWav audio format

Waveform Audio File Format, aka WAV or WAVE, a typically uncompressed audio format.

Support refers to this format's use in the audio element, not other conditions.

Resources: Wikipedia article.
Parent feature: Audio element.
other2.33.2812443.599
otherWOFF - Web Open Font Format

Compressed TrueType/OpenType font that contains information about the font's source.

Browser-specific notes:
#1: Reported to be supported in some modified versions of the Android 4.0 browser.

Resources: Mozilla hacks blog post.
Parent feature: @font-face Web fonts.
rec4.45595.143.699
html5Session history management

Method of manipulating the user's browser's session history in JavaScript using history.pushState, history.replaceState and the popstate event.

Older iOS versions and Android 4.0.4 claim support, but implementation is too buggy to be useful.

Resources: Introduction to history management, MDN Web Docs - Manipulating the browser history, Demo page, History.js polyfill, has.js test, WebPlatform Docs.
ls4.2551064499
html5hidden attribute

The hidden attribute may be applied to any element, and effectively hides elements similar to display: none in CSS.

The hidden state can be easily overridden with a CSS display property set to anything other than none.

Resources: Article on hidden attribute.
ls456115.14499
html5PageTransitionEvent

Fired at the Window when the page's entry in the session history stops being the current entry. Includes the pageshow and pagehide events.

Whether or not a document was/will be cached by a back-forward cache is indicated by event.persisted property where developers can opt to reload the page if needed.

Resources: MDN Web Docs - pageshow, HTML onpageshow Event Attribute, Back/forward cache: web-exposed behaviour, Back-forward cache on Android, web.dev - Back/forward cache.
ls2.3541154299
otherMP3 audio format

Popular lossy audio compression format.

Support refers to this format's use in the audio element, not other conditions.

Browser-specific notes:
#1: Partial support in older Firefox refers to being limited to certain operating systems.

Resources: Wikipedia.
Parent feature: Audio element.
other2.3449442299
domdefer attribute for external scripts

The boolean defer attribute on script elements allows the external JavaScript file to run when the DOM is loaded, without delaying page load first.

Browser-specific notes:
#1: Partial support in older IE refers to a buggy implementation (see issue).
#2: Deferred scripts may run after DOMContentLoaded - https://bugzilla.mozilla.org/show_bug.cgi?id=688580.

Resources: MDN Web Docs - Script Attributes, has.js test, WebPlatform Docs, async vs defer attributes.
ls35810543199
css3CSS font-feature-settings

Method of applying advanced typographic and language-specific font features to supported OpenType fonts.

Whenever possible, font-variant shorthand property or an associated longhand property, font-variant-ligatures, font-variant-caps, font-variant-east-asian, font-variant-alternates, font-variant-numeric or font-variant-position should be used. This property is a low-level feature designed to handle special cases where no other way to enable or access an OpenType font feature exists. In particular, this CSS property shouldn't be used to enable small caps.

Browser-specific notes:
#1: From Gecko 2.0 (Firefox 4.0) to Gecko 14.0 (Firefox 14.0) included, Gecko supported an older syntax, slightly different from the modern one: https://hacks.mozilla.org/2010/11/firefox-4-font-feature-support/.
#2: Partial support in older Chrome versions refers to lacking support in Mac OS X.

Resources: Demo pages (IE/Firefox only), Mozilla hacks article, Detailed tables on accessibility support, WebPlatform Docs, MDN Web Docs - font-feature-settings, OpenType layout feature tag registry, Syntax for OpenType features in CSS (Adobe Typekit Help).
rec1259.348109.153499
html5Range input type

Form field type that allows the user to select a value using a slider widget.

Currently all Android browsers with partial support hide the slider input field by default. However, the element can be styled to be made visible and usable.

Resources: Polyfill for Firefox, Cross-browser polyfill, Tutorial, has.js test, WebPlatform Docs, rangeslider.js polyfill, MDN web docs, Tutorial.
ls4.254103.142399
cssletter-spacing CSS property

Controls spacing between characters of text (i.e. "tracking" in typographical terms). Not to be confused with kerning.

Browser-specific notes:
#1: Truncates or rounds fractional portions of values.

Resources: MDN Web Docs - CSS letter-spacing.
rec4.443096.14299
html5Attributes for form submission

Attributes for form submission that may be specified on submit buttons. The attributes are: formaction, formenctype, formmethod, formnovalidate, and formtarget.

Resources: Article describing each attribute.
ls4515105.14499
jsTyped Arrays

JavaScript typed arrays provide a mechanism for accessing raw binary data much more efficiently. Includes: Int8Array, Uint8Array, Uint8ClampedArray, Int16Array, Uint16Array, Int32Array, Uint32Array, Float32Array & Float64Array.

Includes support for ArrayBuffer objects.

Browser-specific notes:
#1: IE10 (and IE 10&11 mobile) does not support Uint8ClampedArray.
#2: Does not support Float64Array.

Resources: MDN Web Docs - Typed arrays, Polyfill for this feature is available in the core-js library.
Parent feature: ECMAScript 2015 (ES6).
other4571164499
dominput event

The input event is fired when the user changes the value of an <input> element, <select> element, or <textarea> element. By contrast, the "change" event usually only fires after the form control has lost focus.

Browser-specific notes:
#1: Doesn't fire an input event when deleting text (via Backspace, Delete, Cut, etc.).
#2: Doesn't fire an input event when drag-and-dropping text into an <input> or <textarea>.
#3: <select> doesn't fire input events. See MS Edge bug and Firefox bug.
#4: Doesn't fire an input event when (un)checking a checkbox or radio button, or when changing the selected file(s) of an <input type="file">. See Chrome bug, WebKit bug #149398, WebKit bug #190223 and Firefox bug.
#5: Doesn't fire an input event when (un)checking a checkbox or radio button. See MS Edge bug.
#6: Doesn't fire an input event when changing the selected file(s) of an <input type="file">. See WebKit bug #149398 and WebKit bug #204292.

Resources: Specification for `<select>` elements firing the `input` event, MDN Web Docs - input event.
ls4.4.313.2667913.144999
css:optional CSS pseudo-class

The :optional pseudo-class matches form inputs (<input>, <textarea>, <select>) which are not :required.

Browser-specific notes:
#1: Does not match non-required <select>s.

Resources: HTML specification for `:optional`, MDN Web Docs - CSS :optional, JS Bin testcase.
Parent feature: Form validation.
wd2.35151054499
html5progress element

Method of indicating a progress state.

Browser-specific notes:
#1: does not support "indeterminate" <progress> elements.

Resources: CSS-Tricks article, MDN Web Docs - Element progress, Dev.Opera article, Examples of progress and meter elements.
ls4.4881064699
jsECMAScript 5 Strict Mode

Method of placing code in a "strict" operating context.

Browser-specific notes:
#1: Partial support in older Safari refers to strict mode still accepting a lot of JS that should be considered invalid.

Resources: Information page, Article with test suite.
Parent feature: ECMAScript 5.
other35131064499
html5Form validation

Method of setting required fields and field types without requiring JavaScript. This includes preventing forms from being submitted when appropriate, the checkValidity() method as well as support for the :invalid, :valid, and :required CSS pseudo-classes.

Browser-specific notes:
#1: Partial support refers to lack of notice when form with required fields is attempted to be submitted.
#2: Partial support in IE10 mobile refers to lack of warning when blocking submission.
#3: Partial support in Opera Mini refers to only supporting the CSS pseudo classes.

Resources: WebPlatform Docs, WebKit Blog: HTML Interactive Form Validation.
ls4.4.310.3101010.14499
domasync attribute for external scripts

The boolean async attribute on script elements allows the external JavaScript file to run when it's available, without delaying page load first.

Browser-specific notes:
#1: Using script.async = false; to maintain execution order for dynamically-added scripts isn't supported in Safari 5.0.

Resources: MDN Web Docs - Script attributes, Demo, has.js test, async vs defer attributes.
ls358105.143.699
css3CSS Repeating Gradients

Method of defining a repeating linear or radial color gradient as a CSS image.

Firefox 10+, Chrome 26+ and Opera 11.6+ also support the new "to (side)" syntax.

Resources: MDN Web Docs - CSS repeating linear gradient.
Parent feature: CSS Gradients.
cr4.4726106.141699
dommatchMedia

API for finding out whether or not a media query applies to the document.

Resources: matchMedia.js polyfill, MDN Web Docs - matchMedia, MDN Web Docs - Using matchMedia, WebPlatform Docs.
wd359105.14699
html5Pattern attribute for input fields

Allows validation of an input field based on a given regular expression pattern.

Browser-specific notes:
#1: Partial support refers to not displaying a message for invalid patterns.
#2: Safari browsers support the pattern attribute but will still allow forms to be submitted if the pattern is incorrect. See the form validation data for details.

Resources: Site with common sample patterns, MDN Web Docs - input element: pattern attribute.
Parent feature: Form validation.
ls4.4.310.3101010.14499
jsWeb Sockets

Bidirectional communication technology for web apps.

Reported to be supported in some Android 4.x browsers, including Sony Xperia S, Sony TX and HTC.

Browser-specific notes:
#1: Partial support in older browsers refers to the websockets implementation using an older version of the protocol and/or the implementation being disabled by default (due to security issues with the older protocol).
#2: Partial support in older browsers refers to lacking support for binary data.

Resources: Details on newer protocol, Wikipedia, has.js test, WebPlatform Docs, MDN Web Docs - WebSockets API.
ls4.461610741199
domreadonly attribute of input and textarea elements

Makes the form control non-editable. Unlike the disabled attribute, readonly form controls are still included in form submissions and the user can still select (but not edit) their value text.

Browser-specific notes:
#1: Readonly inputs of type datetime-local, date, month, and week can still be edited by pressing the Up or Down arrow keys on the keyboard while the input is focused.
#2: Text cannot be selected directly, but is possible by first selecting any text around the field.

Resources: WHATWG HTML specification for the readonly attribute of the `<textarea>` element, MDN Web Docs - readonly attribute.
ls2.372665.14499
securityStrict Transport Security

Declare that a website is only accessible over a secure connection (HTTPS).

The HTTP header is 'Strict-Transport-Security'.

Browser-specific notes:
#1: IE 11 added support in an update on June 9, 2015.

Resources: Chromium article, MDN Web Docs - Strict Transport Security, OWASP article.
other4.4741274499
canvasWebGL - 3D Canvas graphics

Method of generating dynamic 3D graphics using JavaScript, accelerated through hardware.

WebGL support is dependent on GPU support and may not be available on older devices. This is due to the additional requirement for users to have up to date video drivers.

Note that WebGL is part of the Khronos Group, not the W3C.

Browser-specific notes:
#1: WebGL context is accessed from "experimental-webgl" rather than "webgl".

Resources: Instructions on enabling WebGL, Tutorial, Firefox blog post, Polyfill for IE.
Parent feature: Canvas (basic support).
other12583379842499
jsBlob URLs

Method of creating URL handles to the specified File or Blob object.

Browser-specific notes:
#1: Created blob url can't be used as object or iframe src.

Resources: MDN Web Docs - createObjectURL.
Parent feature: File API.
wd4.4723156.14499
domHTMLElement.innerText

A property representing the text within a DOM element and its descendants. As a getter, it approximates the text the user would get if they highlighted the contents of the element with the cursor and then copied to the clipboard.

This test only checks that the property exists and works correctly in a very simple case.
This blog post by kangax explains the history of this property, gives much more detailed cross-browser compatibility information, and gives a detailed strawman specification for the property.
HTMLElement.innerText is similar to, but has some important differences from, the standard Node.textContent property.

Resources: MDN Web Docs - innerText, WHATWG Compatibility Standard issue #5: spec innerText, Rangy, a JS range and selection library which contains an innerText implementation, Standardizing innerText – Web Incubator Community Group (WICG) discussion.
ls2.34463.244599
html5HTML5 semantic elements

HTML5 offers some new elements, primarily for semantic purposes. The elements include: section, article, aside, header, footer, nav, figure, figcaption, time, mark & main.

Browser-specific notes:
#1: Partial support refers to missing the default styling, as technically the elements are considered "unknown". This is easily taken care of by manually setting the default display value for each tag.
#2: Partial support refers to only the <main> element (added later to the spec) being "unknown", though it can still be used and styled.

Resources: Workaround for IE, Alternate workaround, has.js test, Chrome Platform Status: `<time>` element.
ls4.4726126.142199
html5sandbox attribute for iframes

Method of running external site pages with reduced privileges (e.g. no JavaScript) in iframes.

Resources: Chromium blog article, MSDN article, WebPlatform Docs.
ls2.24.2410542899
jsFileReaderSync

Allows files to be read synchronously in Web Workers.

Resources: MDN Web Docs - FileReaderSync.
Parent feature: FileReader API.
wd4.46151064899
jsBlob constructing

Construct Blobs (binary large objects) either using the BlobBuilder API (deprecated) or the Blob constructor.

Partial support refers to only supporting the now deprecated BlobBuilder to create blobs.

Resources: MDN Web Docs - BlobBuilder, MDN Web Docs - Blobs.
Parent feature: File API.
wd12562010641399
svgSVG SMIL animation

Method of using animation elements to animate SVG images.

Browser-specific notes:
#1: Partial support in older Safari versions refers to not working in HTML files or CSS background images.

Resources: Examples on SVG WOW, MDN Web Docs - animation with SMIL, JS library to support SMIL in SVG, has.js test, Polyfill for SMIL animate events on SVG.
rec3657964499
css3CSS Flexible Box Layout Module

Method of positioning elements in horizontal or vertical stacks. Support includes all properties prefixed with flex, as well as display: flex, display: inline-flex, align-content, align-items, align-self, justify-content and order.

Most partial support refers to supporting an older version of the specification or an older syntax.

Browser-specific notes:
#1: Only supports the old flexbox specification and does not support wrapping.
#2: Only supports the 2012 syntax.
#3: Does not support flex-wrap, flex-flow or align-content properties.
#4: Partial support is due to large amount of bugs present (see known issues).

Resources: Flexbox CSS generator, Article on using the latest spec, Tutorial on cross-browser support, Examples on how to solve common layout problems with flexbox, A Complete Guide to Flexbox, Flexbox playground and code generator, Flexbugs: Repo for flexbox bugs, 10up Open Sources IE 8 and 9 Support for Flexbox, Ecligrid - Mobile first flexbox grid system, The Difference Between Width and Flex-Basis.
cr4.492912942899
jsOnline/offline status

Events to indicate when the user's connected (online and offline events) and the navigator.onLine property to see current status.

"online" does not always mean connection to the internet, it can also just mean connection to some network.

Early versions of Chrome and Safari always reported "true" for navigator.onLine.

Browser-specific notes:
#1: Seems to support navigator.onLine but not online/offline events.
#2: IE8 only supports the online/offline events on document.body, rather than window.
#3: Desktop Firefox responds to the status of its "Work Offline" mode. If not in that mode, navigator.onLine is always true, regardless of the actual network connectivity status. See bug for details.
#4: Safari 7.0 supports only the event listener on window, and not on document.body.

Resources: MDN Web Docs - NavigatorOnLine.onLine.
ls2.381497.144199
domNode.compareDocumentPosition()

Compares the relative position of two nodes to each other in the DOM tree.

Browser-specific notes:
#1: The spec requires that comparisons of nodes in different documents, and comparisons where at least one node is not in any document, must arbitrarily (but consistently) set either the DOCUMENT_POSITION_PRECEDING or DOCUMENT_POSITION_FOLLOWING bit in the result. These browser versions don't set either bit in some such cases.
#2: Sets neither the DOCUMENT_POSITION_DISCONNECTED bit nor the DOCUMENT_POSITION_IMPLEMENTATION_SPECIFIC bit in some cases when comparing nodes in different documents or when comparing a node which is not in any document.

Resources: MDN Web Docs - Node.compareDocumentPosition.
ls4.410309104499
css3CSS3 Border images

Method of using images for borders.

Note that both the border-style and border-width must be specified (not set to none or 0) for border-images to work.

Browser-specific notes:
#1: Has a bug where border-image incorrectly overrides border-style. See test case, WebKit bug, discussion.
#2: Partial support refers to not supporting border-image-repeat: space.
#3: Partial support refers to supporting the shorthand syntax, but not the individual properties (border-image-source, border-image-slice, etc).
#4: Partial support refers to not supporting border-image-repeat: round.

Resources: WebPlatform Docs, MDN Web Docs - Border image.
cr1259.315.4561415.47.25099
jsrequestAnimationFrame

API allowing a more efficient way of running script-based animation, compared to traditional methods using timeouts. Also covers support for cancelAnimationFrame.

Browser-specific notes:
#1: Partial support refers to lacking cancelAnimationFrame support.
#2: Supports webkitCancelRequestAnimationFrame rather than webkitCancelAnimationFrame.

Resources: Blog post, Mozilla Hacks article, WebPlatform Docs, MDN Web Docs - requestAnimationFrame.
ls4.4724106.142399
css3CSS background-position edge offsets

Allows CSS background images to be positioned relative to the specified edge using the 3 to 4 value syntax. For example: background-position: right 5px bottom 5px; for positioning 5px from the bottom-right corner.

Resources: MDN Web Docs - background-position, Basic information.
cr4.47259741399
jsChannel messaging

Method for having two-way communication between browsing contexts (using MessageChannel).

Browser-specific notes:
#1: Supported in Firefox behind the dom.messageChannel.enabled flag. Reported to not work in web workers before version 41.

Resources: An Introduction to HTML5 web messaging, MDN Web Docs - Channel Messaging API.
Parent feature: Cross-document messaging.
ls4.45410544199
css3TTF/OTF - TrueType and OpenType font support

Support for the TrueType (.ttf) and OpenType (.otf) outline font formats in @font-face.

Browser-specific notes:
#1: Partial support in IE9+ refers to the fonts only working when set to be "installable". Support for this is tracked here.

Resources: What is the status of TTF support in Internet Explorer?, OTF Specification.
Parent feature: @font-face Web fonts.
other2.24.24123.143.599
jsPage Visibility

JavaScript API for determining whether a document is visible on the display.

Resources: MDN Web Docs - Page Visibility, SitePoint article, Demo.
rec125733106.151899
domKeyboardEvent.location

A KeyboardEvent property that indicates the location of the key on the input device. Useful when there are more than one physical key for the same logical key (e.g. left or right "Control" key; main or numpad "1" key).

Browser-specific notes:
#1: Only supports KeyboardEvent.keyLocation from an older draft of the DOM Level 3 Events spec instead.

Resources: MDN Web Docs - location.
wd4.483096.141599
css3CSS3 Overflow-wrap

Allows lines to be broken within words if an otherwise unbreakable string is too long to fit. Currently mostly supported using the word-wrap property.

Partial support refers to requiring the legacy name "word-wrap" (rather than "overflow-wrap") to work.

Resources: WebPlatform Docs, Bug on Firefox support, MDN Web Docs - CSS overflow-wrap.
cr4.4723186.144999
domMutation Observer

Method for observing and reacting to changes to the DOM. Replaces MutationEvents, which is deprecated.

When changing the innerHTML content of a node containing a single CharacterData node, resulting in a single-but-different CharacterData child node, WebKit browsers consider this a characterData mutation of the child CharacterData node, while other browsers consider it a childList mutation of the parent node.

Resources: MutationObserver from MDN, Polyfill.
ls4.4727116.141499
jsHigh Resolution Time API

Method to provide the current time in sub-millisecond resolution and such that it is not subject to system clock skew or adjustments. Called using performance.now().

The timestamp is not actually high-resolution. To mitigate security threats such as Spectre, browsers currently round the result to varying degrees.

Browser-specific notes:
#1: In Firefox 55-57, the privacy.resistFingerprinting preference can be enabled to change the precision to 100ms.
#2: In Firefox, the privacy.resistFingerprinting preference can be enabled to change the precision to 100ms or the value of privacy.resistFingerprinting.reduceTimerPrecision.microseconds, whichever is larger.
#3: In Firefox 58-59, the privacy.reduceTimerPrecision preference is enabled by default and defaults to 20us.
#4: In Firefox, the privacy.reduceTimerPrecision preference is enabled by default and defaults to 2ms.

Resources: MDN Web Docs - Performance.now, HTML5Rocks article, SitePoint article, Demo.
rec4.492410846099
jsFileReader API

Method of reading the contents of a File or Blob object into memory.

Browser-specific notes:
#1: Does not support readAsBinaryString, but readAsBinaryString can be polyfilled.

Resources: MDN Web Docs - FileReader, WebPlatform Docs.
Parent feature: File API.
wd36612643.699
jscrypto.getRandomValues()

Method of generating cryptographically random values.

Resources: MDN Web Docs - crypto.getRandomValues.
rec4.4711126.142199
domfocusin & focusout events

The focusin and focusout events fire just before the element gains or loses focus, and they bubble. By contrast, the focus and blur events fire after the focus has shifted, and don't bubble.

In browsers that don't support these events, one alternative is to use a capture phase event listener for the focus and/or blur events.

Resources: MDN Web Docs - focusin, MDN Web Docs - focusout, Mozilla Bug 687787 - Add support for DOM3 focusin/focusout.
wd451565.145299
domclassList (DOMTokenList)

Method of easily manipulating classes on elements, using the DOMTokenList object.

Browser-specific notes:
#1: Does not have support for classList on SVG or MathML elements.
#2: Does not support the second parameter for the toggle method.
#3: Does not support multiple parameters for the add() & remove() methods.
#4: Does not support assign to classList or the replace() method.

Resources: Mozilla Hacks article, Polyfill script, WebPlatform Docs, SitePoint article, Demo using classList, MDN Web Docs - Element.classList.
ls4.472812742699
otherWebVTT - Web Video Text Tracks

Format for marking up text captions for multimedia resources.

WebVTT must be used with the <track> element.

Browser-specific notes:
#1: Firefox 31 - 54 lacks support for the ::cue pseudo-element. See bug #865395..
#2: Firefox 55+ implements ::cue but still lacks support for the ::cue(<selector>) pseudo-element with selector. See bug #1321489..

Resources: Getting Started With the Track Element, An Introduction to WebVTT and track, MDN Web Docs - WebVTT.
Parent feature: Video element.
cr4.472310645599
domdisabled attribute of the fieldset element

Allows disabling all of the form control descendants of a fieldset via a disabled attribute on the fieldset element itself.

Browser-specific notes:
#1: Text inputs that are descendants of a disabled fieldset appear disabled but the user can still interact with them. See IE bug #962368..
#2: File inputs that are descendants of a disabled fieldset appear disabled but the user can still interact with them. See IE bug #817488..

Resources: JS Bin Testcase/Demo.
ls4.46201264499
securityTLS 1.2

Version 1.2 of the Transport Layer Security (TLS) protocol. Allows for data/message confidentiality, and message authentication codes for message integrity and as a by-product message authentication.

Resources: Wikipedia article about TLS 1.2.
other12552911742799
jsconst

Declares a constant with block level scope.

Browser-specific notes:
#1: const is recognized, but treated like var (no block scope, can be overwritten).
#2: const does not have block scope.
#3: Only recognized when NOT in strict mode.
#4: Supported correctly in strict mode, otherwise supported without block scope.
#5: Not supported in for-in and for-of loops.

Resources: Variables and Constants in ES6, MDN Web Docs - const.
other1251149121153699
domCustomEvent

A DOM event interface that can carry custom application-defined data.

Not supported in some versions of Android's old WebKit-based WebView.

Browser-specific notes:
#1: While a window.CustomEvent object exists, it cannot be called as a constructor. Instead of new CustomEvent(...), you must use e = document.createEvent('CustomEvent') and then e.initCustomEvent(...).
#2: There is no window.CustomEvent object, but document.createEvent('CustomEvent') still works.

Resources: MDN Web Docs - CustomEvent, Polyfill based on the MDN snippet, EventListener polyfill which includes a CustomEvent polyfill.
ls4.4615126.141199
domKeyboardEvent.getModifierState()

KeyboardEvent method that returns the state (whether the key is pressed/locked or not) of the given modifier key.

Resources: MDN Web Docs - getModifierState, WebKit feature request bug.
wd4.410.330910.141599
jsIndexedDB

Method of storing data client-side, allows indexed database queries.

Browser-specific notes:
#1: Partial support in IE 10 & 11 refers to a number of subfeatures not being supported. Edge does not support IndexedDB inside blob web workers. See issue.
#2: Partial support in Safari 7.1 – 9.3 refers to seriously buggy behavior as well as a lack of support in WebViews.
#3: Partial support refers to a bug in Safari 14.1.1 that causes indexedDB to not load when the browser is initially opened.

Resources: Mozilla Hacks article, Polyfill for browsers supporting WebSQL, has.js test, WebPlatform Docs.
rec4.41524791541699
securityContent Security Policy 1.0

Mitigate cross-site scripting attacks by only allowing certain sources of script, style, and other resources.

The standard HTTP header is Content-Security-Policy which is used unless otherwise noted.

Browser-specific notes:
#1: Supported through the X-Content-Security-Policy header.
#2: Supported through the X-WebKit-CSP header.

Resources: HTML5Rocks article, CSP Examples & Quick Reference, MDN Web Docs - Content Security Policy.
cr4.472512742399
jsInternationalization API

Locale-sensitive collation (string comparison), number formatting, and date and time formatting.

Resources: MDN Web Docs - Internationalization, The ECMAScript Internationalization API, Working With Intl, WebKit tracking bug, Firefox for Android tracking bug.
other4.41024111042999
css3Font unicode-range subsetting

This @font-face descriptor defines the set of Unicode codepoints that may be supported by the font face for which it is declared. The descriptor value is a comma-delimited list of Unicode range (<urange>) values. The union of these ranges defines the set of codepoints that serves as a hint for user agents when deciding whether or not to download a font resource for a given text run.

Partial support indicates that unnecessary code-ranges are downloaded by the browser - see browser test matrix.

Browser-specific notes:
#1: Can be enabled in Firefox using the layout.css.unicode-range.enabled flag.

Resources: MDN Web Docs - CSS unicode-range, Safari CSS Reference: unicode-range, Web Platform Docs: unicode-range, Demo.
cr1251036171054499
html5wbr (word break opportunity) element

Represents an extra place where a line of text may optionally be broken.

Resources: MDN Web Docs - Element wbr.
ls2.354123.24299
css3CSS background-repeat round and space

Allows CSS background images to be repeated without clipping.

Browser-specific notes:
#1: IE9 does not appear to render "background-repeat: round" correctly.

Resources: MDN Web Docs - background-repeat, CSS-Tricks article on background-repeat.
cr4.473210744999
css3calc() as CSS unit value

Method of allowing calculated values for length units, i.e. width: calc(100% - 3em).

Support can be somewhat emulated in older versions of IE using the non-standard expression() syntax.

Due to the way browsers handle sub-pixel rounding differently, layouts using calc() expressions may have unexpected results.

Browser-specific notes:
#1: Partial support in Android Browser 4.4 refers to the browser lacking the ability to multiply and divide values.
#2: Partial support in IE9 refers to the browser crashing when used as a background-position value.
#3: Partial support in IE10/IE11 refers to calc not working properly with various use cases mentioned in known issues.

Resources: Mozilla Hacks article, MDN Web Docs - calc, WebPlatform Docs.
cr125726126.141699
jsResource Timing (basic support)

Method to help web developers to collect complete timing information related to resources on a document.

For newer Resource Timing APIs search for specific methods or see the sub-features under PerformanceResourceTiming API.

Browser-specific notes:
#1: Can be enabled in Firefox using the dom.enable_resource_timing flag.
#2: Partial support in Safari refers to being limited to OSX 10.12+.

Resources: Demo, Blog post, SitePoint article.
cr4.411251011.143599
css3ch (character) unit

Unit representing the width of the character "0" in the current font, of particular use in combination with monospace fonts.

Browser-specific notes:
#1: IE supports the ch unit, but unlike other browsers its width is that specifically of the "0" glyph, not its surrounding space. As a result, 3ch for example is shorter than the width of the string "000" in IE.

Resources: Blog post on using ch units, Various uses for the ch unit.
cr4.47271274299
jsUser Timing API

Method to help web developers measure the performance of their applications by giving them access to high precision timestamps.

Resources: SitePoint article, HTML5Rocks article, Polyfill, Demo, UserTiming.js polyfill.
rec4.41125101143899
dommatches() DOM method

Method of testing whether or not a DOM element matches a given selector. Formerly known (and largely supported with prefix) as matchesSelector.

Partial support refers to supporting the older specification's "matchesSelector" name rather than just "matches".

Resources: MDN Web Docs - Element matches, WebPlatform Docs.
ls125834157.153499
domLink type "noreferrer"

Links with rel="noreferrer" set do not send the request's "referrer" header. This prevents the destination site from seeing what URL the user came from.

Browser-specific notes:
#1: Only supported in IE11 in later versions of Windows 10 (creators update).

Resources: Blog post on rel="noreferrer".
ls2.341613543399
cssCSS initial value

A CSS value that will apply a property's initial value as defined in the CSS specification that defines the property.

Browser-specific notes:
#1: Not supported on quotes property.

Resources: MDN Web Docs - CSS initial, CSS Tricks article.
cr2.344123.241999
html5Form attribute

Attribute for associating input and submit buttons with a form.

Resources: Input attribute specification, Article on usage.
ls3510165.14499
css3Viewport units: vw, vh, vmin, vmax

Length units representing a percentage of the current viewport dimensions: width (vw), height (vh), the smaller of the two (vmin), or the larger of the two (vmax).

Browser-specific notes:
#1: Partial support in IE9 refers to supporting "vm" instead of "vmin".
#2: Partial support refers to not supporting the "vmax" unit.
#3: Partial support in iOS7 is due to buggy behavior of the "vh" unit (see workarounds: 1, 2).

Resources: Blog post, Polyfill, Buggyfill - Polyfill that fixes buggy support, Back-Forward issue blog post.
cr4.4826166.141999
jsFile API

Method of manipulating file objects in web applications client-side, as well as programmatically selecting them and accessing their data.

Browser-specific notes:
#1: Does not have FileReader support.
#2: Does not support the File constructor.

Resources: MDN Web Docs - Using Files, WebPlatform Docs, Polyfill.
wd4.4.31038791042899
jsWeb Cryptography

JavaScript API for performing basic cryptographic operations in web applications.

Many browsers support the crypto.getRandomValues() method, but not actual cryptography functionality under crypto.subtle.

Browser-specific notes:
#1: Support in IE11 is based on an older version of the specification.
#2: Support in Safari before version 11 was using the crypto.webkitSubtle prefix.
#3: In Edge 12-18, Web Crypto was not supported in Web Workers and Service Workers.

Resources: The History and Status of Web Crypto API, Microsoft Research JavaScript Cryptography Library, Cross-browser cryptography library, Polyfill by Netflix with partial support, PKI.js - another crypto library for Public Key Infrastructure applications, Test suite for various algorithms/methods, Web Cryptography API shim for IE11 and Safari - set of bugfixes and workarounds of prefixed api implementations, MDN Web Docs - Web Crypto API.
rec1251137791143499
cssCSS writing-mode property

Property to define whether lines of text are laid out horizontally or vertically and the direction in which blocks progress.

Browser-specific notes:
#1: Internet Explorer supports different values from an earlier version of the spec, which originated from SVG.
#2: Supported in Firefox under the layout.css.vertical-text.enabled flag.

Resources: MDN Web Docs - CSS writing-mode, Chrome Platform Status.
rec1251148121154199
jsServer-sent events

Method of continuously sending data from a server to the browser, rather than repeatedly requesting it (EventSource interface, used to fall under HTML5).

Resources: HTML5 Rocks tutorial, has.js test, Polyfill.
ls4.4467954699
cssCSS font-stretch

If a font has multiple types of variations based on the width of characters, the font-stretch property allows the appropriate one to be selected. The property in itself does not cause the browser to stretch to a font.

Resources: MDN Web Docs - font-stretch, CSS Tricks article.
rec12510.3489115999
cssHigh-quality kerning pairs & ligatures

When used in HTML, the unofficial text-rendering: optimizeLegibility CSS property enables high-quality kerning and ligatures in certain browsers. Newer browsers have this behavior enabled by default.

The text-rendering property is specified in SVG, though behavior there is not related to kerning pairs & ligatures.

Browser-specific notes:
#1: Partial support in Android browser versions is due to a serious bug where text-rendering: optimizeLegibility causes custom web fonts to not render.

Resources: MDN Web Docs - CSS text-rendering, CSS Tricks article.
unoff4.44.241854399
jsPromisesother4.4.3833127.142999
jsConstraint Validation API

API for better control over form field validation. Includes support for checkValidity(), setCustomValidity(), reportValidity() and validation states.

Browser-specific notes:
#1: Does not support reportValidity.
#2: Does not support validity.tooShort. See also support for minlength..
#3: Does not support validity.badInput.

Resources: MDN article on constraint validation, `reportValidity()` ponyfill.
ls1251040171045199
html5srcdoc attribute for iframes

Override the content specified in the src attribute (if present) with HTML content within the attribute.

Resources: MDN Web Docs - iframe, Srcdoc Polyfill, Article.
ls4.462079642599
html5meter elementls4.410.3813641699
css:indeterminate CSS pseudo-class

The :indeterminate pseudo-class matches indeterminate checkboxes, indeterminate <progress> bars, and radio buttons with no checked button in their radio button group.

Browser-specific notes:
#1: Doesn't match radio buttons whose radio button group lacks a checked radio button.
#2: Doesn't support the <progress> element.
#3: Doesn't match indeterminate <progress> bars.

Resources: HTML specification for `:indeterminate`, MDN Web Docs - CSS :indeterminate, EdgeHTML issue 7124038 - `:indeterminate` pseudo-class doesn't match radio buttons, Mozilla Bug 885359 - Radio groups without a selected radio button should have `:indeterminate` applying, WebKit Bug 156270 - `:indeterminate` pseudo-class should match radios whose group has no checked radio, JS Bin testcase.
wd12510.3397910.145199
cssCrisp edges/pixelated images

Scales images with an algorithm that preserves edges and contrast, without smoothing colors or introducing blur. This is intended for images such as pixel art. Official values that accomplish this for the image-rendering property are crisp-edges and pixelated.

Note that prefixes apply to the value (e.g. -moz-crisp-edges), not the image-rendering property.

Browser-specific notes:
#1: Supported using the non-standard value -webkit-optimize-contrast.
#2: Internet Explorer accomplishes support using the non-standard declaration -ms-interpolation-mode: nearest-neighbor.
#3: Supports the crisp-edges value, but not pixelated.
#4: Supports the pixelated value, but not crisp-edges.
#5: Only works on <img>, not CSS backgrounds or <canvas>.
#6: Only works on <img> and CSS backgrounds, _not_ <canvas>.

Resources: MDN Web Docs - CSS Image rendering, HTML5Rocks article, Firefox bug #856337: Implement image-rendering: pixelated, Chrome bug #317991: Implement image-rendering:crisp-edges.
cr1251041791049399
domXMLHttpRequest advanced features

Updated functionality to the original XHR specification including things like file uploads, transfer progress information and the ability to send FormData. Previously known as XMLHttpRequest Level 2, these features now appear simply in the XMLHttpRequest spec.

See also support for the FormData API and its sub-features, which received more updates since the original XHR Level 2 specification was released.

Browser-specific notes:
#1: Partial support refers to not supporting json as responseType.
#2: Partial support refers to not supporting .timeout and .ontimeout.
#3: Partial support refers to not supporting blob as responseType.
#4: Partial support refers to missing some newer FormData functionality (search for specific APIs for details).

Resources: MDN Web Docs - FormData, Polyfill for FormData object, WebPlatform Docs.
ls12511507911.154799
html5Details & Summary elements

The <details> element generates a simple no-JavaScript widget to show/hide element contents, optionally by clicking on its child <summary> element.

Browser-specific notes:
#1: Enabled in Firefox through the dom.details_element.enabled flag.
#2: 'toggle' event is not supported.
#3: <summary> is not keyboard accessible.
#4: Some versions of Safari display smaller font-size than intended when using rem units with system fonts. (See: https://colloq.io/blog/safaris-detailssummary-rem-font-size-issue).

Resources: jQuery fallback script, Fallback script, HTML5 Doctor article, has.js test, WebPlatform Docs, Bug on Firefox support, Details Element Polyfill.
ls411.336791244999
svgSVG vector-effect: non-scaling-stroke

The non-scaling-stroke value for the vector-effect SVG attribute/CSS property makes strokes appear as the same width regardless of any transformations applied.

Other values for the vector-effect attribute/property are currently at risk of being removed from the specification as they are not being developed by browser vendors.

Resources: MDN Docs article on vector-effect, Firefox implementation bug for other values, Chromium implementation bug for other values.
cr4.4515795.141599
cssCSS :read-only and :read-write selectors

:read-only and :read-write pseudo-classes to match elements which are considered user-alterable.

Browser-specific notes:
#1: Supports selector only for input and textarea fields, but not for contenteditable.

Resources: CSS Tricks article, MDN :read-only, MDN Web Docs - CSS :read-write, Selectors Level 4 § The Mutability Pseudo-classes: :read-only and :read-write, Firefox feature request bug.
ls12593613947899
svgSVG fragment identifiers

Method of displaying only a part of an SVG image by defining a view ID or view box dimensions as the file's fragment identifier.

Browser-specific notes:
#1: Partial support refers to support inside <img> but not as CSS background-image.
#2: Partial support refers to lack of external fragments support.

Resources: Blog post, WebKit support bug.
cr12511.3501211.151599
cssRebeccapurple color

The new color added in CSS Color Module Level 4.

Browser-specific notes:
#1: Only works in IE11 for Windows 10, not older versions of Windows.

Resources: Codepen blog post.
cr4.483812743399
jsdocument.currentScript

document.currentScript returns the <script> element whose script is currently being processed.

Resources: Polyfill (IE 6-10 only).
ls4.48291284499
html5Reversed attribute of ordered lists

This attribute makes an ordered list number its items in descending order (large to small), instead of ascending order (small to large; the default). The order that the list items are displayed in is not affected.

Resources: HTML5 Doctor article on <ol> element attributes (including reversed).
ls4.4620796.141899
jsECMAScript 2015 (ES6)

Support for the ECMAScript 2015 specification. Features include Promises, Modules, Classes, Template Literals, Arrow Functions, Let and Const, Default Parameters, Generators, Destructuring Assignment, Rest & Spread, Map/Set & WeakMap/WeakSet and many more.

As ES6 refers to a huge specification and browsers have various levels of support, "Supported" means at least 95% of the spec is supported. "Partial support" refers to at least 10% of the spec being supported. For full details see the Kangax ES6 support table.

Browser-specific notes:
#1: Notable partial support in IE11 includes (at least some) support for const, let, block-level function declaration, typed arrays, Map, Set and WeakMap.
#2: Does not support tail call optimization.
#3: Only has partial Symbol support and partial support for RegExp.prototype properties.

Resources: ES6 New features: overview and comparisons, Exploring ES6 (book), Polyfill for all possible ES2015 features is available in the core-js library.
other1251051791055499
cssCSS Filter Effects

Method of applying filter effects using the filter property to elements, matching filters available in SVG. Filter functions include blur, brightness, contrast, drop-shadow, grayscale, hue-rotate, invert, opacity, sepia and saturate.

Note that this property is significantly different from and incompatible with Microsoft's older "filter" property.

Browser-specific notes:
#1: Supported in Firefox under the layout.css.filters.enabled flag.
#2: Supported in MS Edge under the "Enable CSS filter property" flag.
#3: Partial support in Firefox before version 34 only implemented the url() function of the filter property.
#4: Partial support refers to supporting filter functions, but not the url function.

Resources: Demo, HTML5Rocks article, Filter editor, Filter Playground.
wd1259.353799.17.23599
domChildNode.remove()

DOM node method to remove the node itself from the document.

Resources: MDN Web Docs - ChildNode.remove.
ls4.4724136.142399
otherHTTP/2 protocol

Networking protocol for low-latency transport of content over the web. Originally started out from the SPDY protocol, now standardized as HTTP version 2.

HTTP/2 is only supported over TLS (HTTPS). See also the precursor of HTTP/2, the SPDY protocol, which has been deprecated and removed from most browsers, in favor of HTTP/2.

Browser-specific notes:
#1: Partial support in Internet Explorer refers to being limited to Windows 10.
#2: Partial support in Safari refers to being limited to OS X 10.11 El Capitan and newer.
#3: Only supports HTTP/2 if the server supports protocol negotiation via ALPN.

Resources: Wikipedia article about HTTP/2, Browser support test.
other125951791155399
canvasCanvas blend modes

Method of defining the effect resulting from overlaying two layers on a Canvas element.

Resources: Blog post.
Parent feature: Canvas (basic support).
cr4.4730136.142099
jsEncrypted Media Extensions

The EncryptedMediaExtenstions API provides interfaces for controlling the playback of content which is subject to a DRM scheme.

Browser-specific notes:
#1: Only supports the older event-based specification.

Resources: HTML5rocks article, Wikipedia article, Encrypted Media Extensions API on MDN.
rec12511.342121253899
css3CSS Feature Queries

CSS Feature Queries allow authors to condition rules based on whether particular property declarations are supported in CSS using the @supports at rule.

See also the CSS.supports() DOM API.

Resources: MDN Web Docs - CSS @supports, @supports in Firefox, Test case, WebPlatform Docs.
cr4.492812942299
jslet

Declares a variable with block level scope.

Browser-specific notes:
#1: Supports a non-standard version that can only be used in script elements with a type attribute of application/javascript;version=1.7. As other browsers do not support these types of script tags this makes support useless for cross-browser support.
#2: Requires the ‘Experimental JavaScript features’ flag to be enabled.
#3: Only supported in strict mode.
#4: let bindings in for loops are incorrectly treated as function-scoped instead of block scoped.
#5: let variables are not bound separately to each iteration of for loops.

Resources: Variables and Constants in ES6, MDN Web Docs - let.
Parent feature: ECMAScript 2015 (ES6).
other1251149121154499
jsES6 Number

Extensions to the Number built-in object in ES6, including constant properties EPSILON, MIN_SAFE_INTEGER, and MAX_SAFE_INTEGER, and methods isFinite, isInteger, isSafeInteger, and isNaN.

Browser-specific notes:
#1: Partial refers to only supporting the isFinite, isInteger, and isNaN methods.
#2: Partial refers to supporting the same as #1 and the addition of the EPSILON property.
#3: Partial refers to supporting all new features except the isSafeInteger method.
#4: Partial refers to only supporting the isFinite and isNaN methods.

Resources: New number and Math features in ES6, Polyfill for those features is available in the core-js library.
Parent feature: ECMAScript 2015 (ES6).
other12593412943299
css:default CSS pseudo-class

The :default pseudo-class matches checkboxes and radio buttons which are checked by default, <option>s with the selected attribute, and the default submit button (if any) of a form.

Whether <option selected> matches :default (per the spec) was not tested since <select>s and <option>s are generally not styleable, which makes it hard to formulate a test for this.

Browser-specific notes:
#1: Does not match <input type="checkbox" checked> or <input type="radio" checked>.
#2: Does not match the default submit button of a form.

Resources: HTML specification for `:default`, MDN Web Docs - CSS :default, WebKit bug 156230 - `:default` CSS pseudo-class should match checkboxes+radios with a `checked` attribute, JS Bin testcase.
wd12510.3517910.15499
domHTML templates

Method of declaring a portion of reusable markup that is parsed but not rendered until cloned.

Browser-specific notes:
#1: Does not support Document.importNode on templates, nested templates or elements that contain templates.
#2: Does not support Node.cloneNode on templates, nested templates or elements that contain templates.
#3: Safari 6.2 has the same partial support as Safari 7.1.

Resources: web.dev - HTML's New template Tag, Polyfill script, Template element on MDN.
ls4.493515942299
jsURL API

API to retrieve the various parts that make up a given URL from a given URL string.

See also URLSearchParams.

Browser-specific notes:
#1: Allows objects to be created via URL constructor, but instances do not have the expected url properties.

Resources: MDN Web Docs - URL interface, MDN Web Docs - URL constructor, Polyfill for this feature is available in the core-js library.
ls4.4.3832127.142699
css:placeholder-shown CSS pseudo-class

Input elements can sometimes show placeholder text as a hint to the user on what to type in. See, for example, the placeholder attribute in HTML5. The :placeholder-shown pseudo-class matches an input element that is showing such placeholder text.

For support of styling the actual placeholder text itself, see CSS ::placeholder.

Browser-specific notes:
#1: Partial support in Firefox refers to using the non-standard :-moz-placeholder name rather than :placeholder-shown.
#2: Partial support in IE refers to using the non-standard :-ms-input-placeholder name rather than :placeholder-shown.

Resources: WebKit commit, Firefox bug.
wd12594779955199
css3CSS3 font-kerning

Controls the usage of the kerning information (spacing between letters) stored in the font. Note that this only affects OpenType fonts with kerning information, it has no effect on other fonts.

Browsers with support for font feature settings can also set kerning value.

Browser-specific notes:
#1: Disabled by default, can be enabled using preference layout.css.font-features.enabled - defaulting to true on Nightly and Aurora only.

Resources: MDN Web Docs - CSS font-kerning.
cr4.4.31233799.143499
css3CSS3 object-fit/object-position

Method of specifying how an object (image or video) should fit inside its box. object-fit options include "contain" (fit according to aspect ratio), "fill" (stretches object to fill) and "cover" (overflows box but maintains ratio), where object-position allows the object to be repositioned like background-image does.

Browser-specific notes:
#1: Partial support in Safari refers to support for object-fit but not object-position.
#2: Partial support in Edge refers to object-fit only supporting <img> (see this comment).

Resources: Dev.Opera article, WebPlatform Docs, object-fit-images Polyfill for IE & Edge, MDN (object-fit), MDN (object-position).
cr4.4.31032791043699
domKeyboardEvent.key

A KeyboardEvent property whose value is a string identifying the key that was pressed. Covers character keys, non-character keys (e.g. arrow keys), and dead keys.

"Some key events, or their values, might be suppressed by the IME in use". On mobile (virtual keyboard), for every key Blink and WebKit based browsers report "Unidentified", Gecko reports "Process".

Browser-specific notes:
#1: Partial support refers to these versions of Firefox returning "MozPrintableKey" for all character keys.
#2: Has non-standard key identifiers and incorrect behaviour with AltGraph.

Resources: MDN Web Docs - key, Chrome tracking bug, WebKit feature request bug, Spec listing all key string values, Edge bug report, shim-keyboard-event-key: shim for non-standard key identifiers for IE & Edge.
wd12510.3517910.152999
css::placeholder CSS pseudo-element

The ::placeholder pseudo-element represents placeholder text in an input field: text that represents the input and provides a hint to the user on how to fill out the form. For example, a date-input field might have the placeholder text YYYY-MM-DD to clarify that numeric dates are to be entered in year-month-day order.

Partial support refers to using alternate names:.
::-webkit-input-placeholder for Chrome/Safari/Opera (Chrome issue #623345).
::-ms-input-placeholder for Edge (also supports webkit prefix).

Browser-specific notes:
#1: Firefox 18 and below supported the :-moz-placeholder pseudo-class rather than the ::-moz-placeholder pseudo-element.

Resources: CSS-Tricks article with all prefixes, CSSWG discussion, MDN Web Docs - CSS ::-moz-placeholder, Mozilla Bug 1069012 - unprefix :placeholder-shown pseudo-class and ::placeholder pseudo-element, MDN web docs - ::placeholder.
wd12510.3577910.17.25199
jsObject.values method

The Object.values() method returns an array of a given object's own enumerable property values.

Resources: Object.values() on MDN Web Docs, Polyfill, Polyfill for this feature is available in the core-js library.
other12510.3541410.16.24799
jsWebRTC Peer-to-peer connections

Method of allowing two users to communicate directly, browser to browser using the RTCPeerConnection API.

The legacy Edge browser also offers a compatible implementation known as ObjectRTC. See Object RTC for support details for that API.

Browser-specific notes:
#1: BlackBerry 10 recognizes RTCPeerConnection but real support is unconfirmed.
#2: Edge does not support RTCDataChannel. See API Catalogue.

Resources: WebRTC Project site, Plug-in for support in IE & Safari, Introducing WebRTC 1.0 and interoperable real-time communications in Microsoft Edge.
wd125115679116.24499
jsWeb Audio API

High-level JavaScript API for processing and synthesizing audio.

Not all browsers with support for the Audio API also support media streams (e.g. microphone input). See the getUserMedia/Streams API data for support for that feature.

Firefox versions < 25 support an alternative, deprecated audio API.

Chrome support went through some changes as of version 36.

Resources: Polyfill to support Web Audio API in Firefox, WebPlatform Docs, Polyfill to enable Web Audio API through Firefox Audio Data api or flash, MDN Web Docs - Web Audio API.
rec12514.5341214.142599
jsTextEncoder & TextDecoder

TextEncoder encodes a JavaScript string into bytes using the UTF-8 encoding and returns the resulting Uint8Array of those bytes. TextDecoder does the reverse.

Browser-specific notes:
#1: Not available in Web Workers in Firefox 19.

Resources: MDN Web Docs - TextEncoder, WebKit Bug 160653 - Support TextEncoder & TextDecoder APIs.
ls12510.3387910.142099
jsES6 Generators

ES6 Generators are special functions that can be used to control the iteration behavior of a loop. Generators are defined using a function* declaration.

Resources: Exploring JS chapter on generators, MDN article on the `function*` declaration.
Parent feature: ECMAScript 2015 (ES6).
other1251039131042699
cssCSS background-blend-mode

Allows blending between CSS background images, gradients, and colors.

Browser-specific notes:
#1: Partial in Safari refers to not supporting the hue, saturation, color, and luminosity blend modes.
#2: Chrome 46 has some serious bugs with multiply, difference, and exclusion blend modes.

Resources: codepen example, Blog post, Demo.
cr12510.3477910.143099
cssCSS all property

A shorthand property for resetting all CSS properties except for direction and unicode-bidi.

Resources: MDN Web Docs - CSS all, Resetting styles using `all: unset`, WebKit bug 116966: [css3-cascade] Add support for `all` shorthand property.
rec4.4.39.337799.142799
css:in-range and :out-of-range CSS pseudo-classes

If a temporal or number <input> has max and/or min attributes, then :in-range matches when the value is within the specified range and :out-of-range matches when the value is outside the specified range. If there are no range constraints, then neither pseudo-class matches.

Note that <input type="range"> can never match :out-of-range because the user cannot input such a value, and if the initial value is outside the range, the browser immediately clamps it to the minimum or maximum (as appropriate) bound of the range.

Browser-specific notes:
#1: Opera Mini correctly applies style on initial load, but does not correctly update when value is changed.
#2: :in-range also incorrectly matches temporal and number inputs which don't have min or max attributes. See Edge bug, Chrome bug, WebKit bug.
#3: :in-range and :out-of-range incorrectly match inputs which are disabled or readonly. See Edge bug, Mozilla bug, WebKit bug, Chrome bug.

Resources: MDN Web Docs - CSS :out-of-range, WHATWG HTML specification for `:in-range` and `:out-of-range`.
wd12510.3537910.155099
domindeterminate checkbox

Indeterminate checkboxes are displayed in a state which is distinct both from being checked or being unchecked. They are commonly used in hierarchical checkboxes to indicate that only some of the checkbox's descendants are checked.

Indeterminacy does not affect a checkbox's checkedness state. It merely affects how the checkbox is displayed.

Resources: CSS-Tricks article, iOS versions below 12 don't support indeterminate checkboxes (WebKit Bug 160484).
ls4.412.2286643.699
cssCSS unset value

A CSS value that's the same as "inherit" if a property is inherited or "initial" if a property is not inherited.

Resources: MDN Web Docs - CSS unset, Resetting styles using `all: unset`, WebKit bug 148614: Add support for the `unset` CSS property value.
rec1259.341139.142799
domCSS.supports() API

The CSS.supports() static method returns a Boolean value indicating if the browser supports a given CSS feature, or not.

See also @supports in CSS.

Browser-specific notes:
#1: Partial support in Presto-based Opera browsers refers to using an older API (window.supportsCSS).
#2: Does not support parentheses-less one-argument version.

Resources: MDN Web Docs - CSS supports(), Demo (Chinese), Native CSS Feature Detection via the @supports Rule, CSS @supports, Article (Chinese).
cr4.496179945599
domPicture element

A responsive images method to control which image resource a user agent presents to a user, based on resolution, media query and/or support for a particular image format.

Browser-specific notes:
#1: Enabled in Chrome through the "experimental Web Platform features" flag in chrome://flags.
#2: Enabled in Opera through the "experimental Web Platform features" flag in opera://flags.
#3: Enabled in Firefox by setting the about:config preference dom.image.picture.enable to true.

Resources: Demo, Tutorial, Read about the use cases, General information about Responsive Images, Blog post on usage, HTML5 Rocks tutorial, Picturefill - polyfill for picture, srcset, sizes, and more.
ls1259.338139.143899
cssCSS will-change property

Method of optimizing animations by informing the browser which elements will change and what properties will change.

Browser-specific notes:
#1: Supported in Firefox behind the layout.css.will-change.enabled flag.

Resources: Detailed article, Blog post, MDN Web Docs - will-change.
cr1259.336799.143699
jsgetUserMedia/Stream API

Method of accessing external device data (such as a webcam video stream). Formerly this was envisioned as the <device> element.

As of Chrome 47, the getUserMedia API cannot be called from insecure origins.

Browser-specific notes:
#1: Blink-based (and some other) browsers support an older version of the spec that does not use srcObject. See Chromium issue 387740.
#2: Supports the older spec's navigator.getUserMedia API, not the newer navigator.mediaDevices.getUserMedia one.
#3: Before iOS 14.3 getUserMedia returned no video input devices in UIWebView or WKWebView, but only directly in Safari.
#4: Does not work in standalone running ("installed") PWAs.

Resources: Technology preview from Opera, WebPlatform Docs, Media Capture functionality in Microsoft Edge, getUserMedia in PWA with manifest on iOS 11, getUserMedia working again in PWA on iOS 13.4.
cr12513.45312116.24299
pngAnimated PNG (APNG)

Like animated GIFs, but allowing 24-bit colors and alpha transparency.

Where support for APNG is missing, only the first frame is displayed.

Resources: Wikipedia, Polyfill using canvas, Chrome extension providing support, Chromium issue (fixed).
cr1258597987.2399
cssCSS Grid Layout (level 1)

Method of using a grid concept to lay out content, providing a mechanism for authors to divide available space for layout into columns and rows using a set of predictable sizing behaviors. Includes support for all grid-* properties and the fr unit.

See also support for subgrids.

Browser-specific notes:
#1: Enabled in Chrome through the "experimental Web Platform features" flag in chrome://flags.
#2: Partial support in IE refers to supporting an older version of the specification.
#3: Enabled in Firefox through the layout.css.grid.enabled flag.
#4: There are some bugs with overflow (1356820, 1348857, 1350925).

Resources: Polyfill based on old spec, Polyfill based on new spec, WebKit Blog post, Css Grid By Example: Everything you need to learn CSS Grid Layout, Mozilla: Introduction to CSS Grid Layout.
cr12510.3581610.16.25499
otherWOFF 2.0 - Web Open Font Format

TrueType/OpenType font that provides better compression than WOFF 1.0.

Browser-specific notes:
#1: Supported only on Safari for Mac OS Sierra, not El Capitan & older.

Resources: Basics about WOFF 2.0, WOFF 2.0 converter.
Parent feature: @font-face Web fonts.
rec1251036141243999
jsES6 Template Literals (Template Strings)

Template literals are string literals allowing embedded expressions using backtick characters (`). You can use multi-line strings and string interpolation features with them. Formerly known as template strings.

Browser-specific notes:
#1: Safari 12 sometimes garbage collects the cached TemplateStrings used by Tagged Template Literals.

Resources: MDN Web Docs - Template literals, ES6 Template Literals in Depth.
Parent feature: ECMAScript 2015 (ES6).
other12512.2411312.143499
jsRest parameters

Allows representation of an indefinite number of arguments as an array.

Browser-specific notes:
#1: Requires the "Experimental JavaScript features" flag to be enabled.

Resources: Rest parameters and defaults.
Parent feature: ECMAScript 2015 (ES6).
other1251047121051599
jsArray.prototype.findIndex

The findIndex() method returns the index of the first element in the array that satisfies the provided testing function.

Resources: MDN article, Polyfill for this feature is available in the core-js library.
other125845127.152599
css3CSS Font Loading

This CSS module defines a scripting interface to font faces in CSS, allowing font faces to be easily created and loaded from script. It also provides methods to track the loading status of an individual font, or of all the fonts on an entire page.

Browser-specific notes:
#1: Can be enabled in Firefox using the layout.css.font-loading-api.enabled flag. Enabled by default in Firefox 41. See this bug.

Resources: Optimizing with font load events.
wd1251035791044199
jsBeacon API

Allows data to be sent asynchronously to a server with navigator.sendBeacon, even after a page was closed. Useful for posting analytics data the moment a user was finished using the page.

Resources: MDN Web Docs - Beacon.
cr12511.3391411.143199
domElement.closest()

DOM method that returns the current element if it matches the given selector, or else the closest ancestor element that matches the given selector, or else null.

Resources: MDN Web Docs - closest, Polyfill.
ls12594115953599
jsString.prototype.includes

The includes() method determines whether one string may be found within another string, returning true or false as appropriate.

Resources: MDN: String.prototype.includes(), Polyfill for this feature is available in the core-js library.
Parent feature: ECMAScript 2015 (ES6).
other12594112944099
securityContent Security Policy Level 2

Mitigate cross-site scripting attacks by only allowing certain sources of script, style, and other resources. CSP 2 adds hash-source, nonce-source, and five new directives.

Browser-specific notes:
#1: Firefox 31-34 is missing the child-src, frame-ancestors, base-uri, and form-action directives.
#2: Firefox 35 is missing the child-src, frame-ancestors, and form-action directives.
#3: Firefox 36-44 is missing the child-src directives.
#4: Chrome 36-38 & Opera 23-25 are missing the child-src, frame-ancestors, base-uri, and form-action directives.
#5: Chrome 39 and Opera 26 are missing the child-src, base-uri, and form-action directives.
#6: Edge has broken nonce support as it ignores nonces on sourced scripts.

Resources: HTML5Rocks article, MDN Web Docs - Content Security Policy.
rec1251040791044599
jsArray.prototype.find

The find() method returns the value of the first item in the array based on the result of the provided testing function.

Resources: MDN article, Polyfill for this feature is available in the core-js library.
other125845157.152599
jsProxy object

The Proxy object allows custom behavior to be defined for fundamental operations. Useful for logging, profiling, object visualization, etc.

Resources: ECMAScript 6 Proxies, MDN Web Docs - Proxy, Experimenting with ECMAScript 6 proxies, Meta programming with ECMAScript 6 proxies, Polyfill for Proxies.
Parent feature: ECMAScript 2015 (ES6).
other1251049121051899
jsArrow functions

Function shorthand using => syntax and lexical this binding.

Resources: ECMAScript 6 features: Arrows, MDN Web Docs - Arrow functions.
Parent feature: ECMAScript 2015 (ES6).
other1251045121052299
jsFetch

A modern replacement for XMLHttpRequest.

Browser-specific notes:
#1: Partial support can be enabled in Firefox with the dom.fetch.enabled flag.
#2: Only available in Chrome and Opera within ServiceWorkers.
#3: Available in Chrome and Opera within Window and Workers by enabling the "Experimental Web Platform Features" flag in chrome://flags.
#4: Firefox <40 is not completely conforming to the specs and does not respect the <base> tag for relative URIs in fetch requests. See bug 1161625.

Resources: Polyfill, Demo, Polyfill (minimal, 500 bytes).
ls12510.3421410.144099
jsnavigator.hardwareConcurrency

Returns the number of logical cores of the user's CPU. The value may be reduced to prevent device fingerprinting or because it exceeds the allowed number of simultaneous web workers.

Browser-specific notes:
#1: WebKit browsers clamp the maximum value returned to 2 on iOS devices and 8 on all others. Disabled in Safari behind the ENABLE_NAVIGATOR_HWCONCURRENCY build option.

Resources: MDN Web Docs - navigator.hardwareConcurrency, Original Proposal, WebKit implementation bug.
ls12510.3371510.144899
domdocument.scrollingElement

document.scrollingElement refers to the element that scrolls the document.

Resources: Polyfill, MDN on scrollingElement.
wd12594414944899
css3CSS Shapes Level 1

Allows geometric shapes to be set in CSS to define an area for text to flow around. Includes properties shape-outside, shape-margin and shape-image-threshold.

Browser-specific notes:
#1: Enabled in Chrome through the "experimental Web Platform features" flag in chrome://flags.
#2: Partially supported in Firefox by enabling "layout.css.shape-outside.enabled" in about:config.

Resources: A List Apart article, Firefox tracking bug.
cr12510.3377910.146299
securityUpgrade Insecure Requests

Declare that browsers should transparently upgrade HTTP resources on a website to HTTPS.

The HTTP header is Content-Security-Policy: upgrade-insecure-requests. Alternatively, the HTML tag is <meta http-equiv="Content-Security-Policy" content="upgrade-insecure-requests">.

Resources: MDN Web Docs - Upgrade Insecure Requests, Demo Website, WebKit feature request bug.
cr12510.3431710.144299
jsES6 classes

ES6 classes are syntactical sugar to provide a much simpler and clearer syntax to create objects and deal with inheritance.

Browser-specific notes:
#1: Requires strict mode. Non-strict mode support is behind the flag 'Enable Experimental JavaScript', disabled by default.

Resources: MDN Web Docs - ES6 classes, Sitepoint deep dive on ES6 classes, List of resources critical of ES6 classes.
Parent feature: ECMAScript 2015 (ES6).
other12594913954599
html5Minimum length attribute for input fields

Declares a lower bound on the number of characters a user can input.

The pattern attribute can be used as an alternative solution for browsers without support.

Resources: W3C usage example, Firefox tracking bug, WebKit feature request bug.
Parent feature: Form validation.
ls12510.3401710.155199
css3CSS Variables (Custom Properties)

Permits the declaration and usage of cascading variables in stylesheets.

Browser-specific notes:
#1: Enabled through the "Experimental Web Platform features" flag in chrome://flags.
#2: Partial support is due to bugs present (see known issues).

Resources: Mozilla hacks article (older syntax), MDN Web Docs - Using CSS variables, Edge Dev Blog post, Polyfill for IE11.
cr1251049161053199
securityChaCha20-Poly1305 cipher suites for TLS

A set of cipher suites used in Transport Layer Security (TLS) protocol, using ChaCha20 for symmetric encryption and Poly1305 for authentication.

Browser-specific notes:
#1: Old versions of Chrome use non-standard code points for ChaCha20-Poly1305 cipher suites.

Resources: Chrome article, SSL/TLS Capabilities of Your Browser by Qualys SSL Labs.
other12511497911.144799
cssMedia Queries: interaction media features

Allows a media query to be set based on the presence and accuracy of the user's pointing device, and whether they have the ability to hover over elements on the page. This includes the pointer, any-pointer, hover, and any-hover media features.

Resources: Potential use cases for script, hover and pointer CSS Level 4 Media Features, Interaction Media Features and their potential (for incorrect assumptions), Polyfill for the `hover` media feature, Chrome support bug.
Parent feature: CSS3 Media Queries.
cr12594112956499
jsArray.prototype.includes

Determines whether or not an array includes the given value, returning a boolean value (unlike indexOf).

Resources: MDN Web Docs, Polyfill for this feature is available in the core-js library.
other12594714954399
jsURLSearchParams

The URLSearchParams interface defines utility methods to work with the query string of a URL.

Browser-specific notes:
#1: Partial support refers to only supporting entries(), keys(), values(), and for...of.

Resources: Easy URL manipulation with URLSearchParams, MDN Web Docs - URLSearchParams, Polyfill for this feature is available in the core-js library, EdgeHTML implementation bug.
ls12510.3491710.154499
html5Subresource Integrity

Subresource Integrity enables browsers to verify that file is delivered without unexpected manipulation.

Browser-specific notes:
#1: Can be enabled via the "Experimental Features" developer menu.

Resources: Subresource Integrity (MDN), SRI generation and browser support test, WebKit feature request bug.
rec12511.345171154399
cssCSS font-variant-numeric

CSS property that provides different ways of displaying numbers, fractions, and ordinal markers.

Resources: MDN Web Docs article.
rec1259.352799.16.23499
cssCSS text-orientation

The CSS text-orientation property specifies the orientation of text within a line. Current values only have an effect in vertical typographic modes (defined with the writing-mode property).

Browser-specific notes:
#1: Can be enabled on firefox via the 'layout.css.vertical-text.enabled' flag in about:config.

Resources: CSS text-orientation on MDN.
rec1251048791454199
cssCase-insensitive CSS attribute selectors

Including an i before the ] in a CSS attribute selector causes the attribute value to be matched in an ASCII-case-insensitive manner. For example, [b="xyz" i] would match both <a b="xyz"> and <a b="XYZ">.

Resources: MDN Web Docs - CSS case-insensitive, JS Bin testcase.
wd12594979954799
domrelList (DOMTokenList)

Method of easily manipulating rel attribute values on elements, using the DOMTokenList object (similar to classList).

Browser-specific notes:
#1: Support for relList on link but not anchor elements.

Resources: MDN Web Docs - DOMTokenList, domtokenlist polyfill.
ls1259651899.23099
otherBrotli Accept-Encoding/Content-Encoding

More effective lossless compression algorithm than gzip and deflate.

Browser-specific notes:
#1: Supported in Chrome and Opera behind the 'Brotli Content-Encoding' flag.
#2: Enabled since 27 May 2016.
#3: Support starting with macOS 10.13 High Sierra.

Resources: Introducing Brotli, Blink's intent to ship, Official code repository, WebKit Bug 154859: Add support for format brotli for HTTP compression.
other1251151151354499
domPassive event listeners

Event listeners created with the passive: true option cannot cancel (preventDefault()) the events they receive. Primarily intended to be used with touch events and wheel events. Since they cannot prevent scrolls, passive event listeners allow the browser to perform optimizations that result in smoother scrolling.

Resources: Improving scroll performance with passive event listeners - Google Developers Updates, Polyfill from the WICG, Original WICG EventListenerOptions repository, JS Bin testcase.
ls1251051161054999
jsDate.prototype.toLocaleDateString

Date method to generate a language sensitive representation of a given date, formatted based on a specified locale and options.

Browser-specific notes:
#1: Does not support the locale or options parameters.
#2: Does not support all locales correctly.
#3: Does not support the timeZone option.

Resources: MDN article on Date​.prototype​.toLocale​Date​String().
other12510.37018129.25699
jsObject.entries

The Object.entries() method creates a multi-dimensional array of key value pairs from the given object.

Resources: MDN article, ES2017 spec-compliant shim for Object.entries, Polyfill for this feature is available in the core-js library.
other12510.3541410.16.24799
domrel=noopener

Ensure new browsing contexts are opened without a useful window.opener.

Resources: Explainer, Gecko/Firefox issue, WebKit/Safari issue.
ls12510.3497910.155299
cssPointer events

This specification integrates various inputs from mice, touchscreens, and pens, making separate implementations no longer necessary and authoring for cross-device pointers easier. Not to be mistaken with the unrelated "pointer-events" CSS property.

Firefox, starting with version 28, provides the 'dom.w3c_pointer_events.enabled' flag to support this specification.

Browser-specific notes:
#1: Partial support in IE10 refers to the lack of pointerenter and pointerleave events.
#2: Firefox support is disabled by default and only supports mouse input. On Windows only, touch can be enabled with the layers.async-pan-zoom.enabled and dom.w3c_touch_events.enabled flags.
#3: Can be enabled with the #enable-pointer-events flag.
#4: Can be enabled under the Experimental Features menu.
#5: element.releasePointerCapture(pointerID) is only partially implemented. Calling it after pointerdown does not result in boundary events (pointerover pointerleave) being dispatched during pointer movements.
#6: Value of pointerevent.buttons is incorrect on pointermove events generated by touches. Is 0 but should be 1.

Resources: Abstraction library for pointer events, PEP: Pointer Events Polyfill, Pointer Event API on MDN, Bugzilla@Mozilla: Bug 822898 - Implement pointer events.
rec12513.25511136.25999
domDOM manipulation convenience methods

jQuery-like methods on DOM nodes to insert nodes around or within a node, or to replace one node with another. These methods accept any number of DOM nodes or HTML strings as arguments. Includes: ChildNode.before, ChildNode.after, ChildNode.replaceWith, ParentNode.prepend, and ParentNode.append.

Browser-specific notes:
#1: Enabled through the "Enable Experimental Web Platform Features" flag in chrome://flags.

Resources: WHATWG DOM Specification for ChildNode, WHATWG DOM Specification for ParentNode, JS Bin testcase, DOM4 polyfill.
ls125105417106.24999
jsIndexedDB 2.0

Improvements to Indexed DB, including getAll(), renaming stores and indexes, and binary keys.

Browser-specific notes:
#1: Missing: IDBKeyRange includes(), renaming stores/indexes, binary keys, IDBObjectStore getKey(), IDBCursor continuePrimaryKey().
#2: Missing: Renaming stores/indexes, binary keys, IDBObjectStore getKey(), IDBCursor continuePrimaryKey().

Resources: Mozilla Hacks: What's new in IndexedDB 2.0?, MDN Web Docs - IndexedDB API.
rec12510.3587910.17.25199
otherFLAC audio format

Popular lossless audio compression format.

Support refers to this format's use in the audio element, not other conditions.

Browser-specific notes:
#1: HTMLMediaElement.canPlayType('audio/flac') returns an empty string.
#2: Only works on ChromeOS.
#3: Supported only on macOS High Sierra or later.

Resources: Wikipedia, Chrome support bug.
Parent feature: Audio element.
other1251156161355199
dom"once" event listener option

Causes an event listener to be automatically removed after it gets invoked, so that it only gets invoked once. Similar to jQuery's $.one() feature.

Resources: Chromium Issue 615384: Support "once" event listener option, JS Bin testcase.
ls125105516106.25099
jsString.prototype.padStart(), String.prototype.padEnd()

The padStart() and padEnd() methods pad the current string with a given string (eventually repeated) so that the resulting string reaches a given length. The pad is applied from the start (left) of the current string for padStart(), and applied from the end (right) of the current string for padEnd().

Resources: MDN Web Docs - padStart(), MDN Web Docs - padEnd(), Polyfill for this feature is available in the core-js library.
other125105715107.24899
jsAsync functions

Async functions make it possible to treat functions returning Promise objects as if they were synchronous.

Browser-specific notes:
#1: Since build 14342 - disabled by default; can be enabled through about:flags.
#2: Async arrow functions are unsupported.

Resources: MDN Web Docs - Async functions, Async functions - making promises friendly.
other125115515116.25299
css3CSS3 tab-size

Method of customizing the width of the tab character. Only effective using 'white-space: pre', 'white-space: pre-wrap', and 'white-space: break-spaces'.

Browser-specific notes:
#1: Partial refers to supporting <integer> but not <length> values.

Resources: MDN Web Docs - CSS tab-size, Firefox bug to unprefix `-moz-tab-size`.
cr12513.4427913.149199
otherWebAssembly

WebAssembly or "wasm" is a new portable, size- and load-time-efficient format suitable for compilation to the web.

Browser-specific notes:
#1: Can be enabled via the javascript.options.wasm in about:config.
#2: Can be enabled via the #enable-webassembly flag.
#3: Can be enabled via the Experimental JavaScript Features flag.
#4: Disabled for Firefox 52 ESR.

Resources: Official site, WebAssembly on MDN, Roadmap and detailed feature support table.
other125115716117.25399
domShadow DOM (V1)

Method of establishing and maintaining functional boundaries between DOM trees and how these trees interact with each other within a document, thus enabling better functional encapsulation within the DOM & CSS.

Browser-specific notes:
#1: Certain CSS selectors do not work (:host > .local-child) and styling slotted content (::slotted) is buggy.
#2: Enabled through the dom.webcomponents.enabled preference in about:config.
#3: Enabled through the dom.webcomponents.shadowdom.enabled preference in about:config.

Resources: Safari implementation bug, Firefox implementation bug, Google Developers - Shadow DOM v1: self-contained web components.
wd125115379106.26399
jsunhandledrejection/rejectionhandled events

The unhandledrejection event is fired when a Promise is rejected but there is no rejection handler to deal with the rejection. The rejectionhandled event is fired when a Promise is rejected, and after the rejection is handled by the promise's rejection handling code.

Resources: MDN article on rejectionhandled, MDN article on unhandledrejection, Chrome sample code, Polyfill for this feature is available in the core-js library.
ls12511.349791156999
cssCSS justify-content: space-evenly

The "space-evenly" value for the justify-content property distributes the space between items evenly. It is similar to space-around but provides equal instead of half-sized space on the edges. Can be used in both CSS flexbox & grid.

Browser-specific notes:
#1: Supported in CSS Grid, but not Flexbox.

Resources: MDN on justify-content property, Edge support bug.
wd125116079118.25299
css#rrggbbaa hex color notation

The CSS Color Module Level 4 defines new 4 & 8 character hex notation for color to include the opacity level.

Only supported in Android WebViews in apps that target Android Pie, due to this issue.

Browser-specific notes:
#1: Can be enabled via the "Experimental web platform features" flag.

Resources: JS Bin testcase.
cr125106279108.24999
cssCSS caret-color

The caret-color property allows the color to be set of the caret (blinking text insertion pointer) in an editable text area.

Resources: WebKit support bug, MDN article.
rec12511.3577911.17.25399
css:focus-within CSS pseudo-classunoff12510.3607910.18.25299
cssCSS font-display

@font-face descriptor font-display that allows control over how a downloadable font renders before it is fully loaded.

Browser-specific notes:
#1: Can be enabled via the layout.css.font-display.enabled flag.

Resources: Google Developers article, MDN Web Docs - font-display, CSS tricks article.
wd12511.3607911.18.25899
csssystem-ui value for font-family

Value for font-family that represents the default user interface font.

Browser-specific notes:
#1: Supported as the -apple-system value (only on macOS and iOS).
#2: Supported as the BlinkMacSystemFont value (only on macOS).

Resources: Firefox support bug, MDN on the font-family property.
wd125115679116.29299
jsJavaScript modules via script tag

Loading JavaScript module scripts (aka ES6 modules) using <script type="module"> Includes support for the nomodule attribute.

Browser-specific notes:
#1: Support can be enabled via about:flags.
#2: Support can be enabled via about:config.
#3: Does not support the nomodule attribute.
#4: A polyfill is available for Safari 10.1/iOS Safari 10.3.
#5: nomodule scripts are not executed, but they're still fetched.

Resources: Intro to ES6 modules, MS Edge blog post, Mozilla hacks article, Firefox support bug, Blog post: Native ECMAScript modules - the first overview, Counterpart ECMAScript specification for import/export syntax, Specification for nomodule attribute, Blog post on using nomodule, Will it double-fetch? Browser behavior with `module` / `nomodule` scripts.
ls125116179118.26099
jsPromise.prototype.finally

When the promise is settled, whether fulfilled or rejected, the specified callback function is executed.

Resources: MDN Web Docs, Polyfill for this feature is available in the core-js library.
other12511.3631811.18.25899
jsJavaScript modules: dynamic import()

Loading JavaScript modules dynamically using the import() syntax.

Browser-specific notes:
#1: Support can be enabled via javascript.options.dynamicImport flag. See bug 1517546.

Resources: Counterpart ECMAScript specification for import() syntax, Blog post: Native ECMAScript modules - dynamic import(), Dynamic import(), Firefox implementation bug, Integration with the HTML specification.
Parent feature: JavaScript modules via script tag.
other12511637911.18.26799
html5Download attribute

When used on an anchor, this attribute signifies that the browser should download the resource the anchor points to rather than navigate to it.

Resources: HTML5Rocks post, IE11 polyfill, Download attribute on MDN.
ls4.413141310.142099
html5Color input type

Form field allowing the user to select a color.

Browser-specific notes:
#1: Supported through WKWebView and Safari but not through UIWebView.

Resources: Polyfill, WebPlatform Docs, MDN web docs.
ls4.412.2201412.142999
canvasPath2D

Allows path objects to be declared on 2D canvas surfaces.

Browser-specific notes:
#1: Does not support the addPath() method.

Resources: MDN article.
Parent feature: Canvas (basic support).
ls125968799.110.14899
cssVariable fonts

OpenType font settings that allows a single font file to behave like multiple fonts: it can contain all the allowed variations in width, weight, slant, optical size, or any other exposed axes of variation as defined by the font designer. Variations can be applied via the font-variation-settings property.

Browser-specific notes:
#1: Works with Experimental Web Platform features enabled.
#2: Requires macOS 10.12 Sierra or later and the following about:config flags to be enabled:.
layout.css.font-variations.enabled,.
gfx.downloadable_fonts.keep_variation_tables.
#3: Requires macOS 10.13 High Sierra or later or Windows 10 1709 "Fall Creators Update" or later.
#4: Does not support the font-weight and font-stretch properties.
#5: Does not support format('truetype-variations'), format('woff-variations'), format('woff2-variations').
#6: Does not support OpenType-CFF2 fonts.
#7: OpenType-CFF2 support requires macOS 10.15 Catalina or later.

Resources: MDN Web docs article, How to use variable fonts in the real world, v-fonts.com - A simple resource for finding and trying variable fonts, Axis-Praxis - Tool & info on variable fonts, Getting started with Variable Fonts on the Frontend (2022).
wd125136717158.27199
domScroll methods on elements (scroll, scrollTo, scrollBy)

Methods to change the scroll position of an element. Similar to setting scrollTop & scrollLeft properties, but also allows options to be passed to define the scroll behavior.

See also the support for the scrollIntoView method.

Browser-specific notes:
#1: Does not support the smooth behavior option.

Resources: MDN article on scrollTo, MDN article on scrollBy.
wd12514.56179148.23698
domIntersectionObserver

API that can be used to understand the visibility and position of DOM elements relative to a containing element or to the top-level viewport. The position is delivered asynchronously and is useful for understanding the visibility of elements and implementing pre-loading and deferred loading of DOM content.

Browser-specific notes:
#1: Enabled in Firefox by setting the about:config preference dom.IntersectionObserver.enabled to true.
#2: isIntersecting property of IntersectionObserverEntry was not implemented, returning undefined.
#3: IntersectionObserver does not trigger its callback in a Chrome background tab.

Resources: MDN Web Docs - Intersection Observer, Polyfill, Google Developers article.
wd12512.2657912.17.25598
cssdisplay: flow-root

The element generates a block container box, and lays out its contents using flow layout. It always establishes a new block formatting context for its contents. It provides a better solution to the most use cases of the "clearfix" hack.

Resources: Mozilla bug report, Chromium bug report, WebKit bug report, Blog post: "The end of the clearfix hack?".
cr125135879137.25398
jsIntl.PluralRules API

API for plural sensitive formatting and plural language rules.

Browser-specific notes:
#1: Intl.PluralRules exists, but methods are not implemented.

Resources: MDN Web Docs: Intl.PluralRules, Google Developers blog: The Intl.PluralRules API.
Parent feature: Internationalization API.
other125136379138.25898
cssCSS Scroll Snap

CSS technique that allows customizable scrolling experiences like pagination of carousels by setting defined snap positions.

Works in the iOS WKWebView, but not UIWebView.

Browser-specific notes:
#1: Partial support in IE10 refers to support limited to touch screens.
#2: Partial support in IE11 documented here.
#4: Partial support in Safari refers to not supporting the none keyword in scroll-snap-points-x, scroll-snap-points-y and scroll-snap-coordinate, and length keywords (top, right, etc.) in scroll-snap-destination and scroll-snap-coordinate.
#5: Supports properties from an older version of the spec.
#6: Partial support in IE & Edge refers to not supporting scroll-snap-coordinate and scroll-snap-destination.
#7: Available behind the "Experimental Web Platform features" feature flag.

Resources: Blog post, MDN Web Docs - CSS Scroll snap points, Polyfill - based on an [older version](https://www.w3.org/TR/2015/WD-css-snappoints-1-20150326/) of the spec, Polyfill - based on the [current version](https://www.w3.org/TR/css-scroll-snap-1/) of the spec, MDN Web Docs - CSS Scroll Snap, A CSS Snap Points based carousel (and lightweight polyfill) .
cr1251169791110.16898
domResize Observer

Method for observing and reacting to changes to sizes of DOM elements.

Browser-specific notes:
#1: Can be enabled via the "Experimental Web Platform Features" flag.

Resources: Google Developers Article, Explainer Doc, Polyfill based on initial specification, Firefox implementation bug, WebKit implementation bug, Polyfill based on latest specification which includes support for observer options.
wd12513.4647913.19.26998
css3text-emphasis styling

Method of using small symbols next to each glyph to emphasize a run of text, commonly used in East Asian languages. The text-emphasis shorthand, and its text-emphasis-style and text-emphasis-color longhands, can be used to apply marks to the text. The text-emphasis-position property, which inherits separately, allows setting the emphasis marks' position with respect to the text.

Some old WebKit browsers (like Chrome 24) support -webkit-text-emphasis, but does not support CJK languages and is therefore considered unsupported.

Browser-specific notes:
#1: Partial support refers to incorrect support for -webkit-text-emphasis-position. These browsers support over and under as values, but not the added left and right values required by the spec.
#2: Can be enabled in Firefox using the layout.css.text-emphasis.enabled flag.

Resources: A javascript fallback for CSS3 emphasis mark., MDN Web Docs - text-emphasis, Chromium bug to unprefix `-webkit-text-emphasis`.
cr125799997.1184698
jsAbortController & AbortSignal

Controller object that allows you to abort one or more DOM requests made with the Fetch API.

Browser-specific notes:
#1: Safari has window.AbortController defined in the DOM but it's just a stub, it does not abort requests at all. The same issue also affects Chrome on IOS and Firefox on IOS because they use the same WebKit rendering engine as Safari.

Resources: Abortable Fetch - Google Developers article, AbortController - MDN Web Docs, AbortSignal - MDN Web Docs.
Parent feature: Fetch.
ls12511.3661612.19.25798
jsflat & flatMap array methodsother1251269791210.16298
cssCSS Environment Variables env()

Usage of environment variables like safe-area-inset-top.

Browser-specific notes:
#1: Supported as constant() instead of env().

Resources: JSFiddle test case, The env() CSS Function - MDN Web Docs, Designing Websites for iPhone X - WebKit Blog.
unoff12511.3697911.110.16598
cssprefers-reduced-motion media query

CSS media query based on a user preference for preferring reduced motion (animation, etc).

prefers-reduced-motion media query also depends on the OS as to whether it is supported.

Resources: WebKit blog post, CSS Tricks article.
wd12510.3747910.111.16398
cssCSS position:sticky

Keeps elements positioned as "fixed" or "relative" depending on how it appears in the viewport. As a result the element is "stuck" when necessary while scrolling.

Any ancestor between the sticky element and its user-scrollable container with overflow computed as anything but visible/clip will effectively prevent sticking behavior.

Browser-specific notes:
#1: Can be enabled in Firefox by setting the about:config preference layout.css.sticky.enabled to true.
#2: Enabled through the "experimental Web Platform features" flag.
#3: Not supported on any table parts - See Firefox bug.
#4: Supported on th elements, but not thead or tr - See Chrome bug.
#5: Do not appear to support sticky table headers.
#6: Supported on th elements, but not thead or tr - See Edge bug.

Resources: HTML5Rocks, MDN Web Docs - CSS position, WebPlatform Docs, Polyfill, Another polyfill, geddski article: Examples and Gotchas.
wd125139191136.25998
otherWebAssembly Import/Export of Mutable Globals

An extension to WebAssembly import and export of mutable global variables.

Resources: Feature extension overview.
Parent feature: WebAssembly.
wd1251274791211.16198
css3CSS3 image-orientation

CSS property used generally to fix the intended orientation of an image. This can be done using 90 degree increments or based on the image's EXIF data using the "from-image" value.

Opening the image in a new tab in Chrome results in the image shown in the orientation according to the EXIF data.

Browser-specific notes:
#1: Partial support in iOS refers to the browser using EXIF data by default, though it does not actually support the property.
#2: Has a bug where the browser is not maintaining the image’s aspect ratio when object-fit: cover and image-orientation: from-image are used together.

Resources: MDN Web Docs - CSS image-orientation, Blog post, Demo (Chinese), Chromium bug #158753: Support for the CSS image-orientation CSS property.
cr12514898913.1152698
otherWebP image format

Image format (based on the VP8 video format) that supports lossy and lossless compression, as well as animation and alpha transparency. WebP generally has better compression than JPEG, PNG and GIF and is designed to supersede them. AVIF and JPEG XL are designed to supersede WebP.

Browser-specific notes:
#1: Partial support refers to not supporting lossless, alpha and animated WebP images.
#2: Partial support refers to not supporting animated WebP images.
#3: Safari 14.0 – 15.6 has full support of WebP, but requires macOS 11 Big Sur or later.

Resources: Official website, Official website FAQ - Which web browsers natively support WebP?, Bitsofcode - Why and how to use WebP images today, WebP decoder and encoder.
other4.21432181646598
domKeyboardEvent.charCode

A legacy KeyboardEvent property that gives the Unicode codepoint number of a character key pressed during a keypress event.

This property is legacy and deprecated.
"Some key events, or their values, might be suppressed by the IME in use". On mobile (virtual keyboard), all keys are reported as 0.

Browser-specific notes:
#1: Does not appear to support the keypress event at all.

Resources: MDN Web Docs - charCode.
unoff2.354944398
canvasWebGL 2.0

Next version of WebGL. Based on OpenGL ES 3.0.

Browser-specific notes:
#1: Can be enabled in Firefox by setting the about:config preference webgl.enable-prototype-webgl2 to true.
#2: WebGL2 context is accessed from "experimental-webgl2" rather than "webgl2".
#3: Can be enabled in Chrome by passing the "--enable-unsafe-es3-apis" flag when starting the browser through the command line.
#4: Can be enabled in the "Experimental Features" developer menu.
#5: Enabled by default for Nightly and Dev Edition.

Resources: Firefox blog post, Getting a WebGL Implementation.
Parent feature: Canvas (basic support).
other125155679157.25198
domWeb Animations API

Lets you create animations that are run in the browser, as well as inspect and manipulate animations created through declarative means like CSS.

Browser-specific notes:
#1: Partial support refers to basic support of element.animate().
#2: Partial support refers to basic support of element.animate() and playback control of AnimationPlayer.
#3: Partial support in Firefox is detailed in Are we animated yet?.
#4: Can be enabled via the "Experimental Features" developer menu.
#5: Does not support composite modes.

Resources: HTML5 Rocks, HTML5 Rocks, Current Firefox status, Polyfill.
wd12513.4848415148198
cssCSS Logical Properties

Logical properties and values provide control of layout through logical, rather than physical, direction and dimension mappings. These properties are writing-mode relative equivalents of their corresponding physical properties.

Browser-specific notes:
#1: Only supports the *-start, and *-end values for margin, border and padding, not the inline/block type values as defined in the spec.
#2: Like #1 but also supports *-before and *-end for *-block-start and *-block-end properties as well as start and end values for text-align.
#3: Does not support the margin-block, margin-inline, padding-block, padding-inline, or any of the inset shorthand properties. Supported in newer Chromium browsers behind the #enable-experimental-web-platform-features flag.
#4: Does not support the border-start-start-radius, border-start-end-radius, border-end-start-radius and border-end-end-radius property.
#5: Does not support the margin-block, margin-inline, padding-block, padding-inline shorthand properties when value is a CSS property. Supported when using a length value.

Resources: MDN Web Docs - CSS -moz-margin-start, MDN Web Docs - CSS -moz-padding-start, MDN - Basic concepts of Logical Properties and Values.
wd12515898915156698
securityTLS 1.3

Version 1.3 (the latest one) of the Transport Layer Security (TLS) protocol. Removes weaker elliptic curves and hash functions.

Browser-specific notes:
#1: Supports a draft of the TLS 1.3 specification, not the final version.
#2: Can be enabled in Firefox by setting the security.tls.version.max pref to "4" in about:config.
#3: Can be enabled in Chrome and Opera via the #tls13-variant flag in chrome://flags or opera://flags.
#4: Partial support in Safari refers to being limited to macOS 10.14 Mojave and later.

Resources: Wikipedia article about TLS 1.3, Chrome support bug.
other12512.270791410.16398
cssCSS math functions min(), max() and clamp()wd12513.4797913.1127598
cssCSS Conical Gradients

Method of defining a conical or repeating conical color gradient as a CSS image.

Browser-specific notes:
#1: Can be enabled via the "Experimental Web Platform Features" flag.
#2: Does not support multi-position color stops.
#3: Can be enabled with the layout.css.conic-gradient.enabled flag.

Resources: Client-side polyfill, Server-side polyfill (PostCSS), Mozilla bug #1175958: Implement conic gradients from CSS Image Values Level 4, MDN Web Docs - conic-gradient().
Parent feature: CSS Gradients.
wd12512.2717912.110.18398
securityReferrer Policy

A policy that controls how much information is shared through the HTTP Referer header. Helps to protect user privacy.

Browser-specific notes:
#1: Browsers initially supported an early draft of the specification which can only use a meta tag and is only compatible with the origin value from the new spec.
#2: Chrome 21-60 did not support the same-origin, strict-origin and strict-origin-when-cross-origin values. (issue 627968).
#3: Older browsers defaulted to no-referrer-when-downgrade which is unsafe. The default has been updated to strict-origin-when-cross-origin which is safe for most use cases. See the revised spec..
#4: Safari's ITP (on by default) downgrades _all_ cross-site subresource request (not page navigation) Referer headers to the page's origin, ignoring the unsafe-url, no-referrer-when-downgrade, and origin-when-cross-origin referrer policies.
#5: Firefox 92+ ignores the unsafe-url, no-referrer-when-downgrade, and origin-when-cross-origin referrer policies, defaulting to strict-origin-when-cross-origin.

Resources: Mozilla security article, A new security header: Referrer Policy.
cr12515858515149298
jsBigInt

Arbitrary-precision integers in JavaScript.

Browser-specific notes:
#1: Can be enabled by setting javascript.options.bigint to "True" in about:config.

Resources: GitHub repository, Blog article from Google Developer, Blog article from Dr. Axel Rauschmayer, MDN Web Docs, Firefox implementation bug.
other125146779149.26898
cssprefers-color-scheme media query

Media query to detect if the user has set their system to use a light or dark color theme.

Support will also depend on whether or not the OS has support for a light/dark theme preference.

Resources: MDN article, Firefox implementation bug, Chromium implementation issue, Web.dev article.
wd12513767912.1126798
css:is() CSS pseudo-class

The :is() (formerly :matches(), formerly :any()) pseudo-class checks whether the element at its position in the outer selector matches any of the selectors in its selector list. It's useful syntactic sugar that allows you to avoid writing out all the combinations manually as separate selectors. The effect is similar to nesting in Sass and most other CSS preprocessors.

Browser-specific notes:
#1: Only supports the deprecated :-webkit-any() pseudo-class.
#2: Only supports the deprecated :-webkit-any() and :matches() pseudo-classes.
#3: Only supports the deprecated :-moz-any() pseudo-class.
#4: Support for :is() can be enabled with the Experimental Web Platform features flag.

Resources: WebKit blog post about adding `:matches()` and other Selectors Level 4 features, Chrome support bug for :is(), MDN Web Docs - CSS :is(), Codepen - Modern tests, JS Bin - Legacy tests.
wd12514888814157898
cssCSS zoom

Method of scaling content while also affecting layout.

Originally implemented only in Internet Explorer.  Note that transform: scale() does not work the same as zoom. If e.g. transform: scale(0.6) is used on the html or body element then it resizes the entire page, showing a minified page with huge white margins around it, whereas zoom: 0.6 scales the *elements* on the page, but not the page itself on which the elements are drawn.

Browser-specific notes:
#1: The -ms-zoom property is an extension to CSS, and can be used as a synonym for zoom in IE8 Standards mode.

Resources: CSS Tricks, Safari Developer Library, Article explaining usage of zoom as the hack for fixing rendering bugs in IE6 and IE7., MDN Web Docs - CSS zoom.
wd2.144124412698
cssCSS revert value

A CSS keyword value that resets a property's value to the default specified by the browser in its UA stylesheet, as if the webpage had not included any CSS. For example, display:revert on a <div> would result in display:block. This is in contrast to the initial value, which is simply defined on a per-property basis, and for display would be inline.

Resources: MDN Web Docs - CSS revert, Firefox feature request bug, Chrome feature request issue.
cr1259.384849.1146798
css3CSS ::marker pseudo-element

The ::marker pseudo-element allows list item markers to be styled or have their content value customized.

Browser-specific notes:
#1: Safari support is limited to color and font-size. See bug 204163.

Resources: Chrome support bug, Firefox support bug, WebKit support bug, MDN Web Docs - CSS ::marker, CSS-Tricks article.
wd12511.3868611.1‒17.0TP146898
cssselector list argument of :not()

Selectors Level 3 only allowed :not() pseudo-class to accept a single simple selector, which the element must not match any of. Thus, :not(a, .b, [c]) or :not(a.b[c]) did not work. Selectors Level 4 allows :not() to accept a list of selectors. Thus, :not(a):not(.b):not([c]) can instead be written as :not(a, .b, [c]) and :not(a.b[c]) works as intended.

Resources: MDN Web Docs - CSS :not, Chrome feature request issue, Firefox feature request bug.
wd125988889158498
css3CSS Gradients

Method of defining a linear or radial color gradient as a CSS image.

Syntax used by browsers with prefixed support may be incompatible with that for proper support.

Support can be somewhat emulated in older IE versions using the non-standard "gradient" filter.

Firefox 10+, Opera 11.6+, Chrome 26+ and IE10+ also support the new "to (side)" syntax.

Browser-specific notes:
#1: Partial support in Opera 11.10 and 11.50 also refers to only having support for linear gradients.
#2: Partial support in Safari and Older Firefox versions refers to not using premultiplied colors which results in unexpected behavior when using the transparent keyword as advised by the spec.
#3: Implements an earlier prefixed syntax as -webkit-gradient.

Resources: Cross-browser editor, Tool to emulate support in IE, WebPlatform Docs.
cr4.4715.4261015.443698
otherWebAssembly Sign Extension Operators

An extension to WebAssembly adding sign-extension operator instructions.

Resources: Feature extension overview.
Parent feature: WebAssembly.
wd12514.5747914.111.16298
otherWebAssembly Non-trapping float-to-int Conversion

An extension to WebAssembly adding floating-point to integer conversion operators which saturate instead of trapping.

Resources: Feature extension overview.
Parent feature: WebAssembly.
wd1251575791511.16498
otherWebAssembly Threads and Atomics

An extension to WebAssembly adding shared memory and atomic memory operations.

Threads support in browsers also includes passive data segments (from the bulk-memory extension).

Resources: Feature extension overview.
Parent feature: WebAssembly.
wd12514.5747914.111.17998
otherWebAssembly Bulk Memory Operations

An extension to WebAssembly adding bulk memory operations and conditional segment initialization.

Passive data segments/conditional segment initialization was also included by browsers with the threads extension.

Resources: Feature extension overview.
Parent feature: WebAssembly.
wd1251575791511.17998
otherWebAssembly Multi-Value

An extension to WebAssembly allowing instructions, blocks and functions to produce multiple result values.

Resources: Feature extension overview.
Parent feature: WebAssembly.
wd12513.2858513.1147898
html5Datalist element

Method of setting a list of options for a user to select in a text field, while leaving the ability to enter a custom value.

While most commonly used on text fields, datalists can also be used on other input types. IE11 supports the element on range fields. Chrome and Opera also support datalists to suggest given values on range, color and date/time fields.

Browser-specific notes:
#1: Partial support refers to a bug where long lists of items are unscrollable resulting in unselectable options.
#2: Partial support in IE refers to significantly buggy behavior (IE11+ does send the input and change events upon selection).
#3: Partial support refers to no support for datalists on non-text fields (e.g. number, range, color).
#4: Partial support in Edge refers to disappearing option elements on focusing the input element via tab.
#5: Supported through WKWebView and Safari but not through UIWebView.
#6: In Firefox, autocomplete must be set to off to make dynamic datalist work due to a bug.

Resources: Mozilla Hacks article, HTML5 Library including datalist support, MDN Web Docs - datalist, WebPlatform Docs, Eiji Kitamura's options demos & tests, Minimal Datalist polyfill w/tutorial, Minimal and library dependency-free vanilla JavaScript polyfill.
ls4.4.312.2697912.1411098
css3CSS Hyphenation

Method of controlling when words at the end of lines should be hyphenated using the "hyphens" property.

Chrome < 55 and Android 4.0 Browser support "-webkit-hyphens: none", but not the "auto" property. It is advisable to set the @lang attribute on the HTML element to enable hyphenation support and improve accessibility.

Browser-specific notes:
#1: Only supported on Android & Mac platforms (and only the "auto" value) for now. See commit & related bug.
#2: Only supported on Android & Mac platforms.

Resources: MDN Web Docs - CSS hyphens, Blog post, WebPlatform Docs, Chromium bug for implementing hyphenation, WebKit bug to unprefix `-webkit-hyphens`.
cr1254.2‒16.017881055.1176.24398
otherMathML

Special tags that allow mathematical formulas and notations to be written on web pages.

Support was added in Chrome 24, but temporarily removed afterwards due to instability.

Browser-specific notes:
#1: Before version 4, Firefox only supports the XHTML notation.
#2: Before version 10, Safari had issues rendering significant portions of the MathML torture test.
#3: Can be enabled via the #enable-experimental-web-platform-features flag in chrome://flags.
#4: Browsers based on Chromium 109+ specifically support MathML Core. While there is significant support overlap with other MathML implementations there are some differences (see details).
#5: Opera's support is limited to a CSS profile of MathML.

Resources: Wikipedia, Cross-browser support script, MDN Web Docs - MathML, MathML torture test, MathML in Chromium Project Roadmap.
rec12551091091021498
domscrollIntoView

The Element.scrollIntoView() method scrolls the current element into the visible area of the browser window. Parameters can be provided to set the position inside the visible area as well as whether scrolling should be instant or smooth.

Browser-specific notes:
#1: Supports scrollIntoView with boolean parameter, but not smooth behavior option.

Resources: MDN Web Docs - scrollIntoView, smooth scroll polyfill : polyfill for smooth behavior option.
wd1255166179168.23698
cssgap property for Flexbox

gap for flexbox containers to create gaps/gutters between flex items.

Resources: Spec discussion, Chrome bug to track implementation, MDN browser compatibility, Workaround using negative margins, Webkit support bug.
Parent feature: CSS Flexible Box Layout Module.
wd12514.5848414.1146398
cssMedia Queries: resolution feature

Allows a media query to be set based on the device pixels used per CSS unit. While the standard uses min/max-resolution for this, some browsers support the older non-standard device-pixel-ratio media query.

Browser-specific notes:
#1: Supports the dpi unit, but does not support dppx or dpcm units.
#2: Firefox before 16 supports only dpi unit, but you can set 2dppx per min--moz-device-pixel-ratio: 2.
#3: Supports the non-standard min/max-device-pixel-ratio.
#4: Does not support x unit (alias for dppx unit), e.g: @media (min-resolution: 2x) { }.

Resources: How to unprefix -webkit-device-pixel-ratio, WebKit Bug 78087: Implement the 'resolution' media query, WHATWG Compatibility Standard: -webkit-device-pixel-ratio, MDN Web Docs - CSS @media resolution, CSS Values and Units Module Level 4 add the `x` unit as an alias for `dppx`., Chrome support 'x' as a resolution unit., Firefox support 'x' as a resolution unit..
Parent feature: CSS3 Media Queries.
cr12541668791610.16298
otherWebAssembly BigInt to i64 conversion in JS API

An extension to the WebAssembly JS API for bidrectionally converting BigInts and 64-bit WebAssembly integer values.

Safari supported BigInt in version 14.1, but did not support BigInt64Array (which is also required for Emscripten's BigInt support) until version 15.

Resources: Feature extension overview.
Parent feature: WebAssembly.
wd12514.5858514.1147898
cssCSS Appearance

The appearance property defines how elements (particularly form controls) appear by default. By setting the value to none the default appearance can be entirely redefined using other CSS properties.

WebKit, Blink, and Gecko browsers also support additional vendor specific values.

Browser-specific notes:
#1: The appearance property is supported with the none value, but not auto.
#2: Microsoft Edge and IE Mobile support this property with the -webkit- prefix, rather than -ms- for interop reasons.
#3: -moz-appearance:none did not remove the dropdown arrow in the select tag.

Resources: CSS Tricks article, Safari implementation bug for unprefixed `appearance`.
wd1253.215.4848415.4148098
otherWebAssembly Reference Types

An extension to WebAssembly allowing opaque references as first-class types, and multiple tables.

Resources: Feature extension overview.
Parent feature: WebAssembly.
wd12515969615177998
cssCSS overflow property

Originally a single property for controlling overflowing content in both horizontal & vertical directions, the overflow property is now a shorthand for overflow-x & overflow-y. The latest version of the specification also introduces the clip value that blocks programmatic scrolling.

Effectively all browsers support the CSS 2.1 definition for single-value overflow as well as overflow-x & overflow-y and values visible, hidden, scroll & auto.

Browser-specific notes:
#1: Does not support the two value overflow shorthand.
#2: Does not support the clip value.

Resources: CSS overflow on MDN, WebKit bug on support for two values syntax, Edge bug on support for two values syntax, WebKit bug on support for clip value.
wd12513.416909016158198
domKeyboardEvent.which

A legacy KeyboardEvent property that is equivalent to either KeyboardEvent.keyCode or KeyboardEvent.charCode depending on whether the key is alphanumeric.

This property is legacy and deprecated.
"Some key events, or their values, might be suppressed by the IME in use". On mobile (virtual keyboard), all keys are reported as 229.

Browser-specific notes:
#1: event.which number for keys does not always match expected value (difference may be dependent on hardware vs. on-screen keyboard?).

Resources: MDN Web Docs - which.
unoff1255495.15297
cssCSS Scroll-behavior

Method of specifying the scrolling behavior for a scrolling box, when scrolling happens due to navigation or CSSOM scrolling APIs.

Browser-specific notes:
#1: Partial support refers to support everything except of Element.scrollIntoView() and not together with pinch viewport.
#2: Supported in Chrome and Opera behind the 'Smooth Scrolling' and/or 'Enable experimental web platform features' flag.
#3: Supported in Safari via the "CSSOM View Smooth Scrolling" Experimental Features flag.

Resources: MDN Web Docs - CSS scroll-behavior, Chrome launch bug , Blog post with demo, iOS / WebKit bug report.
wd12514.515.4617915.48.23697
cssCSS overscroll-behavior

CSS property to control the behavior when the scroll position of a scroll container reaches the edge of the scrollport.

Browser-specific notes:
#1: Supports the precursor version of the property: -ms-scroll-chaining, which works similarly. IE support is limited to Windows 8 & above.
#2: Does not support overscroll-behavior: none; on the body element to prevent the overscroll glow and rubberbanding effects.
#3: Incorrectly treats none as contain see MDN and Edge 18 changelog.
#4: Can be enabled in the "Experimental Features" developer menu.

Resources: Demo, Firefox implementation bug, Google Developers blog post on overscroll-behavior, WebKit support bug, CSS overscroll-behavior on MDN.
wd12514.5166579168.25997
css3CSS font-variant-alternates

Controls the usage of alternate glyphs associated to alternative names defined in @font-feature-values for certain types of OpenType fonts.

Browser-specific notes:
#1: Low-level syntax available in font-feature-settings property equivalent to OpenType features: salt, ss01 through ss20, cv01 through cv99, swsh, cswh, ornm, nalt.
#2: Experimental support available by enabling the layout.css.font-features.enabled flag.

Resources: MDN Web Docs - font-variant-alternates, Chromium support bug.
wd1259.3161111119.1223497
css3CSS3 text-align-last

CSS property to describe how the last line of a block or a line right before a forced line break when text-align is justify.

Browser-specific notes:
#1: In Internet Explorer, the start and end values are not supported.
#2: Enabled through the "Enable Experimental Web Platform Features" flag in chrome://flags.
#3: Enabled through the "Enable Experimental Web Platform Features" flag in opera://flags.

Resources: MDN Web Docs - CSS text-align-last, WebKit support bug.
cr1253.21647791654997
cssselector list argument of :nth-child and :nth-last-child CSS pseudo-classes

The newest versions of :nth-child() and :nth-last-child() accept an optional of S clause which filters the children to only those which match the selector list S. For example, :nth-child(1 of .foo) selects the first child among the children that have the foo class (ignoring any non-foo children which precede that child). Similar to :nth-of-type, but for arbitrary selectors instead of only type selectors.

For support information for just :nth-child() see CSS3 selector support.

Resources: Mozilla Bug 854148 - Support for :nth-child(An+B of sel), :nth-last-child(An+B of sel) pseudo-classes, Chromium Issue 304163: Implement :nth-child(an+b of S) and :nth-last-child(an+b of S) pseudo-classes, MS Edge Platform Status: Under Consideration.
wd1259411111192211397
jsBroadcastChannel

BroadcastChannel allows scripts from the same origin but other browsing contexts (windows, workers) to send each other messages.

Resources: MDN Web Docs - Broadcast Channel, Shim - Broadcast Channel based on Localstorage, Indexeddb or Sockets.
ls1253.215.4547915.47.23897
domDialog element

Method of easily creating custom dialog boxes to display to the user with modal or non-modal options. Also includes a ::backdrop pseudo-element for behind the element.

Browser-specific notes:
#1: Supported in Firefox by enabling dom.dialog_element.enabled in about:config.
#2: Enabled through the "Experimental Web Platform features" flag in chrome://flags.
#3: Enabled through the "Experimental Web Platform features" flag in opera://flags.
#4: Enabled by default in Firefox Nightly only.

Resources: Polyfill.
ls1253.215.4377915.449897
cssCSS Motion Path

Allows elements to be animated along SVG paths or shapes via the offset-path property. Originally defined as the motion-path property.

Browser-specific notes:
#1: Requires the "Experimental Web Platform features" flag to be enabled.

Resources: Blog post, MDN Web Docs - CSS motion-path, Demo, Firefox tracking bug.
wd1253.21646791657297
cssCSS Containment

The CSS contain property lets developers limit the scope of the browser's styles, layout and paint work for faster and more efficient rendering.

Browser-specific notes:
#1: Can be enabled in older Gecko engines with the layout.css.contain.enabled flag in about:config.

Resources: Google Developers article.
rec1253.215.4527915.46.26997
cssCSS color() function

The CSS color() function allows the browser to display colors in any color space, such as the P3 color space which can display colors outside of the default sRGB color space.

For this function to work properly, the device screen and OS also needs to support the color space being used.

Browser-specific notes:
#1: Only supports display-p3 predefined color profile.
#2: Available behind the Experimental Web Platform features flag.
#3: Can be enabled by setting about:config flag layout.css.more_color_4.enabled to true.

Resources: Chromium implementation bug, Firefox implementation bug, WebKit article on using color() with the P3 color space, Color generator that uses color() with the P3 color space.
cr12515108111111152211397
cssCSS Masks

Method of displaying part of an element, using a selected image as a mask.

Browser-specific notes:
#1: Partial support in WebKit/Blink browsers refers to supporting the mask-image and mask-box-image properties, but lacking support for other parts of the spec.
#2: Partial support in Firefox refers to only support for inline SVG mask elements i.e. mask: url(#foo).
#3: Partial support refers to supporting the mask-box-image shorthand but not the longhand properties.
#4: Can be enabled in MS edge behind the "Enable CSS Masking" flag.
#5: Partial support refers to supporting mask-image and mask-size.
#6: Partial support refers to supporting mask-image, mask-size, mask-position, mask-repeat and mask-composite.
#7: Edge also recognizes and supports all the -webkit- prefixed equivalents of the unprefixed properties for site compatibility.

Resources: WebPlatform Docs, HTML5 Rocks article, Firefox implementation bug, Visual test cases, Detailed blog post (via The Internet Archive).
cr1253.215.4412012015.4255397
css:focus-visible CSS pseudo-class

The :focus-visible pseudo-class applies while an element matches the :focus pseudo-class, and the UA determines via heuristics that the focus should be specially indicated on the element (typically via a “focus ring”).

Previously drafted as :focus-ring.

Browser-specific notes:
#1: As :-moz-focusring.
#2: Enabled through the "Experimental Web Platform features" flag in chrome://flags.
#3: Can be enabled via ":focus-visible pseudo-class" in the Experimental Features menu.

Resources: Prototype for `:focus-visible`, Chrome does not support CSS Selectors 4 :focus-visible, Blink: Intent to implement :focus-visible pseudo class., Mozilla Developer Network (MDN) documentation - :-moz-focusring, Bugzilla: Add :focus-visible (former :focus-ring), Bugzilla: implement :focus-visible pseudo-class (rename/alias :-moz-focusring), WebKit bug #185859: [selectors] Support for Focus-Indicated Pseudo-class: `:focus-visible`.
wd1251515.4868615.4148596
otherAAC audio file format

Advanced Audio Coding format, designed to be the successor format to MP3, with generally better sound quality.

Support refers to this format's use in the audio element, not other conditions.

Browser-specific notes:
#1: Partial support in Firefox refers to only supporting AAC in an MP4 container and only when the operating system already has the codecs installed.

Resources: Wikipedia article.
Parent feature: Audio element.
other34129442296
cssLCH and Lab color values

The lch() and lab() color functions are based on the CIE LAB color space, representing colors in a way that closely matches human perception and provides access to a wider spectrum of colors than offered by the usual RGB color space.

Browser-specific notes:
#1: Can be enabled by setting about:config flag layout.css.more_color_4.enabled to true.
#2: Can be enabled via the #enable-experimental-web-platform-features flag in chrome://flags.

Resources: Chrome support bug, Firefox support bug, LCH colors in CSS: what, why, and how?, LCH color picker, MDN article on lch(), MDN article on lab().
cr125154111111152211396
cssCSS Cascade Layers

The @layer at-rule allows authors to explicitly layer their styles in the cascade, before specificity and order of appearance are considered.

Browser-specific notes:
#1: Can be enabled with the "layout.css.cascade-layers.enabled" feature flag under about:config.
#2: Can be enabled in Chrome Canary using the --enable-blink-features=CSSCascadeLayers runtime flag.

Resources: The Future of CSS: Cascade Layers (CSS @layer), Chromium support bug, Firefox support bug, WebKit support bug, Collection of demos.
cr1253.215.4999915.4189796
cssCSS user-select: nonewd1253.254793.16.26996
cssCSS font-palette

The font-palette CSS property allows selecting a palette from a color font. In combination with the @font-palette-values at-rule, custom palettes can be defined.

Resources: Demo, Explainer, Safari 15.4 Beta Release Notes, Firefox support bug.
wd1253.215.410110515.41910796
securityWeb Authentication API

The Web Authentication API is an extension of the Credential Management API that enables strong authentication with public key cryptography, enabling password-less authentication and / or secure second-factor authentication without SMS texts.

Browser-specific notes:
#1: Can be enabled at about:flags.

Edge 13 used an earlier draft syntax. As of Edge 14 the implementation is prefixed and based on the FIDO 2.0 Web APIs.
#2: Can be enabled using the Develop > Experimental Features menu. Currently supports USB-based CTAP & CTAP2 HID devices.
#3: Can be enabled in Settings under the Safari > Advanced > Experimental Features menu.
#4: Partial support refers to FIDO2 devices not working in all operating systems if a PIN is set.
#5: Experimental feature not yet enabled in WKWebView-based browsers including Chrome for iOS.
#6: Partial support refers to TouchID not being supported.

Resources: Web Authentication on MDN Web Docs, Web Authentication and Windows Hello, Guide to Web Authentication.
rec12514.56718131711496
cssCSS Container Query Units

Container Query Units specify a length relative to the dimensions of a query container. The units include: cqw, cqh, cqi, cqb, cqmin, and cqmax.

Browser-specific notes:
#1: Can be enabled with the "Enable CSS Container Queries" feature flag under chrome://flags.
#2: The initial Chrome implementation used an earlier spec draft syntax: q* units.

Resources: Blog post: CSS Container Query Units, CSS Tricks: Container Units Should Be Pretty Handy, Firefox support bug.
wd1253.216105105162011096
cssCSS Container Queries (Size)

Size queries in Container Queries provide a way to query the size of a container, and conditionally apply CSS to the content of that container.

Browser-specific notes:
#1: Can be enabled with the "Enable CSS Container Queries" feature flag under chrome://flags.
#2: The initial Chrome prototype used an earlier draft syntax based on the contain property.
#3: Combining size container queries and table layout inside a multicolumn layout doesn't work.

Resources: Container Queries: a Quick Start Guide, MDN article, Chromium support bug, Collection of demos, WebKit support bug, Firefox support bug, Container Query Polyfill.
wd1253.216106106162011096
cssSmall, Large, and Dynamic viewport units

Viewport units similar to vw and vh that are based on shown or hidden browser UI states to address shortcomings of the original units. Currently defined as the sv* units (svbsvhsvisvmaxsvminsvw), lv* units (lvblvhlvilvmaxlvminlvw), dv* units (dvbdvhdvidvmaxdvmindvw) and the logical vi/vb units.

Browser-specific notes:
#1: Can be enabled via the Experimental Web Platform features flag.

Resources: Blog post explaining the new units, Chromium support bug, Firefox support bug, WebKit support bug, MDN Web Docs - Relative length units based on viewport.
wd1253.215.410810815.42110196
cssCSS Backdrop Filter

Method of applying filter effects (like blur, grayscale or hue) to content/elements below the target element.

Browser-specific notes:
#1: Can be enabled via the "Experimental Web Platform Features" flag.
#2: Currently only supported with the -webkit- prefix (not -ms-).
#3: Can be enabled by setting the layout.css.backdrop-filter.enabled and gfx.webrender.all preference to true in about:config.

Resources: Blog post, MDN Web Docs - CSS backdrop filter, WebKit bug to unprefix `-webkit-backdrop-filter`.
unoff1259.0‒17.01876799‒17.0181210395
html5Drag and Drop

Method of easily dragging and dropping elements on a page, requiring minimal JavaScript.

dataTransfer.items only supported by Chrome.

Currently no browser supports the dropzone attribute.

Firefox supports any kind of DOM elements for .setDragImage. Chrome must have either an HTMLImageElement or any kind of DOM Element attached to the DOM _and within the viewport_ of the browser for .setDragImage.

Browser-specific notes:
#1: Partial support refers to no support for the dataTransfer.files or .types objects.
#2: Partial support refers to not supporting .setDragImage.
#3: Partial support refers to limited supported formats for dataTransfer.setData/getData.
#4: Not supported in Chromium browsers on Android 6 or older.

Resources: HTML5 Doctor article, Shopping cart demo, WebPlatform Docs, Polyfill for setDragImage in IE, iOS/Android shim for HTML 5 drag'n'drop.
ls125154183.143.595
css3Intrinsic & Extrinsic Sizing

Allows for the heights and widths to be specified in intrinsic values using the max-content, min-content, fit-content and stretch (formerly fill) properties.

Prefixes are on the values, not the property names (e.g. -webkit-min-content).

Older webkit browsers also support the unofficial intrinsic value which acts the same as max-content.

Browser-specific notes:
#1: Firefox does not support the height/min-height/max-height properties, only width. See test case, Firefox bug.
#2: Firefox supports the -moz-available keyword rather than stretch. Firefox bug.
#3: Does not support the flex-basis property. See specs, Blink bug, Firefox bug.
#4: Chrome does not yet unprefix stretch (aka fill/fill-available), because the CSSWG is not ready for that yet. See Chromium bug.
#5: While still being affected by note 1, it is only for -moz-fit-content and -moz-available.

Resources: Min-Content tutorial.
wd1251416949416176695
css:has() CSS relational pseudo-class

Select elements containing specific content. For example, a:has(img) selects all <a> elements that contain an <img> child.

Browser-specific notes:
#1: Can be enabled via the Experimental Web Platform features flag.
#2: Supported in Firefox behind the layout.css.has-selector.enabled flag, which is enabled by default since 119 (only Nightly).

Resources: MDN Web Docs - :has, Firefox support bug, Chrome bug to track implementation, WebKit support bug, Using :has() as a CSS Parent Selector and much more.
wd1253.215.410510515.42012195
otherAVIF image format

A modern image format based on the AV1 video format. AVIF generally has better compression than WebP, JPEG, PNG and GIF and is designed to supersede them. AVIF competes with JPEG XL which has similar compression quality and is generally seen as more feature-rich than AVIF.

Browser-specific notes:
#1: Can be enabled in Firefox via the image.avif.enabled pref in about:config.
#2: Supports still images. Animated image sequences are not supported.
#3: Only available on macOS 13 Ventura or later.
#4: Does not support images with noise synthesis.
#5: Supports only still images by default. Animated image sequences can be enabled via the image.avif.sequence.enabled pref in about:config.
#6: Can be enabled via the --enable-features=msEdgeAVIF runtime flag in insider channels.

Resources: Polyfill, AVIF for Next-Generation Image Coding - blog post, Safari support bug, Firefox support bug for image sequence and animation.
other1253.21616.4851213.116.41411395
html5Ping attribute

When used on an anchor, this attribute signifies that the browser should send a ping request the resource the attribute points to.

While still in the WHATWG specification, this feature was removed from the W3C HTML5 specification in 2010.

Browser-specific notes:
#1: Disabled by default for privacy reasons. Can be enabled via the browser.send_pings flag.

Resources: MDN Web Docs - Element ping attribute.
ls4.45151764395
domTouch events

Method of registering when, where and how the interface is touched, for devices with a touch screen. These DOM events are similar to mousedown, mousemove, etc.

Internet Explorer implements Pointer Events specification which supports more input devices than Touch Events one.

There is a library on GitHub that is working toward bringing W3C touch events to IE 10 and 11: https://github.com/CamHenlin/TouchPolyfill.

Removed support in Firefox refers to desktop Firefox only.

Browser-specific notes:
#1: Can be enabled in Firefox using the dom.w3c_touch_events.enabled flag (disabled by default for site compatibility reasons).
#2: Supported on IE11 Mobile for phones with "Windows Phone 8.1 Update".
#3: Can be enabled in Edge using the "Enable touch events" flag under about:flags.

Resources: Detailed support tables, Multi-touch demo, Information on the spec development, Internet Explorer's gesture and touch implementation., Touch polyfill for supporting touch events on Internet Explorer, MDN – Touch events.
rec2.13.222793.145295
cssCSS touch-action property

touch-action is a CSS property that controls filtering of gesture events, providing developers with a declarative mechanism to selectively disable touch scrolling (in one or both axes) or double-tap-zooming.

Browser-specific notes:
#1: Supported in Firefox behind the layout.css.touch_action.enabled flag, Firefox for Windows 8 Touch ('Metro') enabled by default.
#2: IE10+ has already supported these property which are not in standard at present such as double-tap-zoom, cross-slide-x, cross-slide-y.
#3: iOS Safari only supports auto and manipulation.
#4: Not applicable to Firefox platforms that support neither pointer nor touch events.

Resources: 300ms tap delay, gone away, What Exactly Is..... The 300ms Click Delay, MDN Web Docs - CSS touch-action.
rec1251336113.145795
cssCSS image-set

Method of letting the browser pick the most appropriate CSS image from a given set.

Browser-specific notes:
#1: Has very limited support. Only url() is accepted as the image and only x is accepted as a resolution.
#2: Does not support generated images in content. Example: content: image-set(linear-gradient()). Bug 1696314.
#3: Does not support the type() function.
#4: Does not support omitting the resolution.
#5: Does not support using calc() for the resolution.

Resources: Web Platform Tests, Firefox implementation bug, Chromium bug to update and unprefix image-set(), WebKit bug to support type().
wd12514.0‒16.0172111411414172311394
cssCSS Subgrid

Feature of the CSS Grid Layout Module Level 2 that allows a grid-item with its own grid to align in one or both dimensions with its parent grid.

Browser-specific notes:
#1: Available behind the Experimental Web Platform features flag.

Resources: CSS Grid Level 2: Here Comes Subgrid, Why we need CSS subgrid , Firefox support bug, Chromium support bug, Webkit support bug.
cr1253.216411711716247194
dominputmode attribute

The inputmode attribute specifies what kind of input mechanism would be most helpful for users entering content into the form control.

Browser-specific notes:
#1: Support can be enabled via the dom.forms.inputmode flag.

Resources: Demo on Wufoo (old), Everything You Ever Wanted to Know About inputmode (CSS Tricks).
ls12512.266793.19.29594
cssCSS widows & orphans

CSS properties to control when lines break across pages or columns by defining the amount of lines that must be left before or after the break.

Some older WebKit-based browsers recognize the properties, but do not appear to have actual support.

Browser-specific notes:
#1: Supports widows & orphans properties, but due to not supporting CSS multi-columns the support is only for page breaks (for print).

Resources: CSS last-line: Controlling Widows & Orphans, Firefox support bug, codrops article on orphans, codrops article on widows.
rec4.47251074294
domElement.scrollIntoViewIfNeeded()

If the element is fully within the visible area of the viewport, it does nothing. Otherwise, the element is scrolled into view. A proprietary variant of the standard Element.scrollIntoView() method.

Resources: Mozilla Bug 403510 - Implement scrollIntoViewIfNeeded, W3C CSSOM View bug #17152: Support centering an element when scrolling into view..
unoff2.3515795.14294
html5Lazy loading via attribute for images & iframes

The loading attribute on images & iframes gives authors control over when the browser should start loading the resource.

Browser-specific notes:
#1: Firefox only supports lazy loading for images.
#2: Can be enabled in Settings under the Safari > Advanced > Experimental Features menu.
#3: Safari supports lazy image loading. Lazy iframes loading can be enabled under the Experimental Features menu.

Resources: Blog post, Explainer, WebKit support bug, Firefox support bug for lazy loading iframes, Polyfill.
ls12513.415.416.4777915.416.41212192
jsLookbehind in JS regular expressions

The positive lookbehind ((?<= )) and negative lookbehind ((?<! )) zero-width assertions in JavaScript regular expressions can be used to ensure a pattern is preceded by another pattern.

Resources: Blog post on lookbehind assertions, Firefox implementation bug, MDN: Regular Expressions Assertions, Safari implementation bug.
other1253.2‒16.016.462793.116.48.27892
html5OffscreenCanvas

OffscreenCanvas allows canvas drawing to occur with no connection to the DOM and can be used inside workers.

Browser-specific notes:
#1: Can be enabled via the gfx.offscreencanvas.enabled flag. Currently only supports WebGL contexts, not 2D.
#2: Can be enabled via the Experimental canvas features flag.
#3: Partial support refers to only supporting 2D contexts, not WebGL.

Resources: MDN article, WebGL off the main thread - Mozilla Hacks article, Making the whole web better, one canvas at a time. - Article about canvas performance, Firefox support bug, WebKit support bug.
ls1253.2‒16.01769793.11710.110592
jsImport maps

Import maps allow control over what URLs get fetched by JavaScript import statements and import() expressions.

Browser-specific notes:
#1: Can be enabled via the "Experimental Web Platform features" flag at about:flags.
#2: Can be enabled via the dom.importMaps.enabled pref in about:config.

Resources: Proposal information, Using ES modules in browsers with import-maps, Firefox feature request bug, WebKit feature request bug.
unoff1253.2‒16.016.489893.116.41510892
otherWebAssembly SIMD

An extension to WebAssembly adding 128-bit SIMD operations.

Resources: Feature extension overview.
Parent feature: WebAssembly.
wd1253.2‒16.016.491913.116.4168991
cssMedia Queries: Range Syntax

Syntax improvements to make media queries using features that have a "range" type (like width or height) less verbose. Can be used with ordinary mathematical comparison operators: >, <, >=, or <=.

For example: @media (100px <= width <= 1900px) is the equivalent of @media (min-width: 100px) and (max-width: 1900px).

Resources: Syntax improvements in Level 4, Firefox support bug, PostCSS Polyfill, WebKit bug, Media Queries Level 4: Media Query Range Contexts (Media Query Ranges), New syntax for range media queries in Chrome 104.
Parent feature: CSS3 Media Queries.
cr1253.2‒16.016.41041043.116.4206391
jsScreen Wake Lock API

API to prevent devices from dimming, locking or turning off the screen when the application needs to keep running.

Browser-specific notes:
#1: Available behind the #experimental-web-platform-features feature flag.
#2: Available behind the dom.screenwakelock.enabled flag, which is only enabled by default in early Beta or earlier.

Resources: Stay awake with the Screen Wake Lock API, Firefox support bug, WebKit support bug, MDN article about the Screen Wake Lock API.
cr1253.2‒16.016.485903.116.41412691
domDeclarative Shadow DOM

Proposal to allow rendering elements with shadow dom (aka web components) using server-side rendering.

Browser-specific notes:
#1: Uses an older non-standard attribute called shadowroot instead of the standardized shadowrootmode attribute.

Resources: Declarative Shadow DOM - web.dev article, A ponyfill of the Declarative Shadow DOM API, WebKit support bug, Firefox implementation bug.
ls1253.2‒16.016.4901111113.116.42212390
jsServer Timing

Mechanism for web developers to annotate network requests with server timing information.

Browser-specific notes:
#1: Chrome v60-63: An older version of the API is behind the Experimental Web Platform features flag.
#2: Chrome v64: The current API is behind the Experimental Web Platform features flag.
#3: Safari v12.1+: Supports the Server Timing reporting in the Web Inspector Network tab, but not the JavaScript API.

Resources: Demo, Blog post, MDN article on PerformanceServerTiming.
wd1253.2‒16.016.4657912.116.446190
css:dir() CSS pseudo-class

Matches elements based on their directionality. :dir(ltr) matches elements which are Left-to-Right. :dir(rtl) matches elements which are Right-to-Left.

Browser-specific notes:
#1: Can be enabled via the #enable-experimental-web-platform-features flag.

Resources: HTML specification for `:dir()`, MDN Web Docs - CSS :dir, Chrome issue #576815: CSS4 pseudo-class :dir(), WebKit bug #64861: Need support for :dir() pseudo-class, JS Bin testcase.
wd1253.2‒16.016.4911201203.116.4254990
jsCredential Management API

API that provides a programmatic interface to the browser's credential manager. In short, an origin can request a user's credentials to sign them in, or can ask the browser to save credentials on the user's behalf. Both of these requests are user-mediated.

Browser-specific notes:
#1: In Chrome 51 ~ 56, PSL matched credentials are not supported. This means you cannot use credentials set on a.example.com in b.example.com. This is supported since Chrome 57.

Resources: Tutorial by Google, MDN Web Docs - Credential Management API, Live Demo, Sample Code, Spec discussion.
wd1251457793.17.2289
cssCSS text-stroke and text-fill

Method of declaring the outline (stroke) width and color for text.

Does not yet appear in any W3C specification. Was briefly included in a spec as the "text-outline" property, but this was removed.

Browser-specific notes:
#1: Firefox & Edge specifically only support the -webkit-text-stroke property (not using -moz- or -ms- prefix).
#2: Requires the layout.css.prefixes.webkit flag to be enabled.

Resources: Information & workarounds, Live editor, MDN Web Docs - text-stroke.
unoff444793.144989
cssCSS line-clamp

CSS property that will contain text to a given amount of lines when used in combination with display: -webkit-box. It will end with ellipsis when text-overflow: ellipsis is included.

Older (presto-based) versions of the Opera browser have also supported the same effect using the proprietary -o-ellipsis-lastline; value for text-overflow.

Browser-specific notes:
#1: Supported in MS Edge with -webkit- prefix only (not with -ms-).

Resources: CSS Tricks article.
wd2.351479546889
cssCSS print-color-adjust

The print-color-adjust (or -webkit-print-color-adjust as prefixed in WebKit/Blink browsers) property is a CSS extension that can be used to force printing of background colors and images.

Resources: MDN web docs - print-color-adjust, Edge issue with print-color-adjust, Chromium bug with print-color-adjust property.
cr1253.215.4197915.444888
css3CSS text-size-adjust

On mobile devices, the text-size-adjust CSS property allows Web authors to control if and how the text-inflating algorithm is applied to the textual content of the element it is applied to.

Browser-specific notes:
#1: If the viewport size is set using a <meta> element, the -ms-text-size-adjust property is ignored. See MDN.
#2: Old versions of WebKit-based desktop browsers (Chrome<27, Safari<6) suffer from a bug where if -webkit-text-size-adjust is explicitly set to none, instead of ignoring the property, the browsers will prevent the user from zooming in or out on the webpage.

Resources: MDN Web Docs - text-size-adjust, Mozilla Bug #1226116: Unprefix -moz-text-size-adjust.
unoff1255547965288
cssCSS Cross-Fade Function

Image function to create a "crossfade" between images. This allows one image to transition (fade) into another based on a percentage value.

Resources: Firefox bug #546052: Implement cross-fade(), MDN Web Docs - CSS cross-fade(), Chromium bug to unprefix `-webkit-cross-fade()`.
cr4.4101779104287
otherURL Scroll-To-Text Fragment

URL fragment that defines a piece of text to be scrolled into view and highlighted.

Resources: Current Firefox position, Safari's position as of Jan 2020, Firefox support bug, Article about why the `:~:` syntax.
unoff1253.2‒16.016.181833.116.113287
securityTLS 1.1

Version 1.1 of the Transport Layer Security (TLS) protocol.

TLS 1.0 & 1.1 are deprecated in Chrome, Edge, Firefox, Internet Explorer 11, & Safari.

Browser-specific notes:
#1: Firefox 68+ displays a small warning icon in the address bar when connecting over TLS 1.1.
#2: Firefox 78+ displays a full page dismissable warning the first time it connects over TLS 1.1.
#3: Chrome 85+ and Safari for MacOS 13.1+ displays Not secure in the address bar when connecting over TLS 1.1.
#4: Chrome 85+ displays a full page dismissable warning every time a new site connects over TLS 1.1.

Resources: Wikipedia article about TLS 1.1, Modernizing Transport Security - Google Security Blog, Modernizing TLS connections in Microsoft Edge and Internet Explorer 11 - Microsoft Windows Blog, Removing Old Versions of TLS - Mozilla Security Blog, Deprecation of Legacy TLS 1.0 and 1.1 Versions - WebKit Blog.
other1255851113.147887
html5Audio element

Method of playing sound on webpages (without requiring a plug-in). Includes support for the following media properties: currentSrc, currentTime, paused, playbackRate, buffered, duration, played, seekable, ended, autoplay, loop, controls, volume & muted.

Browser-specific notes:
#1: Old Firefox versions were missing support for some properties: loop was added in v11, played in v15, playbackRate in v20.
#2: iOS volume is read-only.

Resources: HTML5 Doctor article, Detailed article on video/audio elements, Demos of audio player that uses the audio element, Detailed article on support, has.js test, WebPlatform Docs, The State of HTML5 Audio.
ls2.33.2493.142086
jsSelection API

API for accessing selected content of a document, including the window.getSelection() method, as well as the selectstart & selectionchange events on document.

See also support for the related DOM range.

Browser-specific notes:
#1: Supports window.getSelection() but no selection events.
#2: Supports selection events, but not window.getSelection().
#3: Does not support the selectstart event.
#4: window.getSelection() may fail on tapping buttons, as the selection is lost right before the listener code runs.
#5: Selection events are supported behind the dom.select_events.enabled flag.

Resources: Firefox support bug.
wd4.451595.145286
domMedia Source Extensions

API allowing media data to be accessed from HTML video and audio elements.

Browser-specific notes:
#1: Partial support in IE11 refers to only working in Windows 8+.
#2: Fully supported only in iPadOS, 13 and later.

Resources: MDN Web Docs - MediaSource, MSDN article, MediaSource demo.
rec4.4.313311289.24286
otherWebM video format

Multimedia format designed to provide a royalty-free, high-quality open video compression format for use with HTML5 video. WebM supports the video codec VP8 and VP9.

Browser-specific notes:
#1: Older browser versions did not support all codecs.
#2: Older Edge versions did not support progressive sources.
#3: Can be enabled in Internet Explorer and Safari for MacOS by manually installing the codecs in the operating system.
#4: Supports VP8 codec used in WebRTC.
#5: Supports VP9 codec used in WebRTC (off by default.).
#6: Supports VP9 WebM used in MediaSource on macOS Big Sur or later..
#7: Safari 14.1 – 15.6 has full support of WebM, but requires macOS 11.3 Big Sur or later.

Resources: Official website, Wikipedia article.
Parent feature: Video element.
other12514.0‒17.017.425791652886
cssCSS Relative colors

The CSS Relative Color syntax allows a color to be defined relative to another color using the from keyword and optionally calc() for any of the color values.

Browser-specific notes:
#1: Can be enabled in Chrome via the #enable-experimental-web-platform-features flag in chrome://flags.

Resources: Dynamic Color Manipulation with CSS Relative Colors, Chromium bug, Firefox support bug, Relative color syntax (blog post).
wd1253.2‒16.016.441191193.116.425212885
cssCSS Reflections

Method of displaying a reflection of an element.

Similar effect can be achieved in Firefox 4+ using the -moz-element() background property.

Resources: WebKit blog post.
unoff2.13.247944284
otherOpus audio format

Royalty-free open audio codec by IETF, which incorporated SILK from Skype and CELT from Xiph.org, to serve higher sound quality and lower latency at the same bitrate.

Support refers to this format's use in the audio element, not other conditions.

For Opera the Linux version may be able to play it when the GStreamer module is up to date and the served mime-type is 'audio/ogg'.

Browser-specific notes:
#1: Supported only when packaged in a CAF file and on macOS High Sierra/iOS 11 or later (constant bit-rate only).
#2: Supported in a WebM container (all bitrates). Has a known regression.
#3: Supported in a WebM container (all bitrates).

Resources: Introduction of Opus by Mozilla, Google's statement about the use of VP8 and Opus codec for WebRTC standard.
Parent feature: Audio element.
other12511.0‒17.017.5331411‒17.017.551584
html5Srcset and sizes attributes

The srcset and sizes attributes on img (or source) elements allow authors to define various image resources and "hints" that assist a user agent to determine the most appropriate image source to display (e.g. high-resolution displays, small monitors, etc).

Browser-specific notes:
#1: Can be enabled in Firefox by setting the about:config preference dom.image.srcset.enabled to true.
#2: Supports the subset of the syntax for resolution switching (using the x descriptor), but not the full syntax that can be used with sizes (using the w descriptor).
#3: Intermittently displays distorted images due to bug present (see known issues).
#4: Does not support min/max expressions.
#5: For JavaScript-generated img elements, loads src first, even if srcset will also make it load another image (see known issues).

Resources: Improved support for high-resolution displays with the srcset image attribute, Blog post on srcset & sizes, MDN: Responsive images, MDN: <img> element.
ls125938161143884
cssBlending of HTML/SVG elements

Allows blending between arbitrary SVG and HTML elements.

Browser-specific notes:
#1: Enabled in Chrome through the "experimental Web Platform features" flag in chrome://flags.
#2: Partial in Safari refers to not supporting the hue, saturation, color, and luminosity blend modes.

Resources: codepen example, Blog post.
cr125841797.153284
domCustom Elements (V1)

One of the key features of the Web Components system, custom elements allow new HTML tags to be defined.

Browser-specific notes:
#1: Supports "Autonomous custom elements" but not "Customized built-in elements" see WebKit bug 182671.
#2: Enabled through the dom.webcomponents.enabled preference in about:config.
#3: Enabled through the dom.webcomponents.customelements.enabled preference in about:config.

Resources: Google Developers - Custom elements v1: reusable web components, customElements.define polyfill, WebKit Blog: Introducing Custom Elements.
ls12510.3677910.16.26384
jsStreams

Method of creating, composing, and consuming streams of data, that map efficiently to low-level I/O primitives, and allow easy composition with built-in backpressure and queuing.

Browser-specific notes:
#1: IE implements a different API than the one from WHATWG.
#2: Only basic read support.
#3: Disabled by default behind the javascript.options.streams and dom.streams.enabled flags.
#4: No support for BYOB ("bring your own buffer") stream readers.
#5: No support for WritableStream.
#6: No support for the pipeTo or pipeThrough methods.

Resources: GitHub repository, ReadableStream on Mozilla Developer Network, Blog article about streams.
ls12510.3898914.11510283
domautocomplete attribute: on & off values

The autocomplete attribute for input elements indicates to the browser whether a value should or should not be autofilled when appropriate.

This support information does not include support for other autocomplete values.

As described in detail below, many modern browsers ignore the off value on certain fields in certain cases intentionally in order to give the user more control over autofilling fields. One example is the use of password managers.

Browser-specific notes:
#1: Partial support refers to ignoring the off value for password fields. see related blog post.
#2: Partial support in Chrome refers to the browser intentionally ignoring autocomplete="off" when the user uses the browser's autofill functionality. see bug.
#3: Partial support in Firefox refers to ignoring autocomplete="off" for login forms. see bug.
#4: Browser does not display previously submitted values as options with on value.
#5: Safari ignores the off value for username, email and password fields.

Resources: MDN Web Docs - autocomplete attribute.
ls2.154179743083
html5Date and time input types

Form field widgets to easily allow users to enter a date, time or both, generally by using a calendar/time input widget. Refers to supporting the following input types: date, time, datetime-local, month & week.

There used to also be a datetime type, but it was dropped from the HTML spec.

Browser-specific notes:
#1: Partial support in Microsoft Edge refers to supporting date, week, and month input types, and not time and datetime-local.
#2: Partial support in iOS Safari refers to not supporting the week input type, nor the min, max or step attributes.
#3: Some modified versions of the Android 4.x browser do have support for date/time fields.
#4: Can be enabled in Firefox using the dom.forms.datetime flag.
#5: Partial support refers to supporting date and time input types, but not datetime-local, month or week.
#6: Partial support refers to not supporting the week and month input type.

Resources: Datepicker tutorial w/polyfill, Polyfill for HTML5 forms, has.js test, WebPlatform Docs, Bug on Firefox support, Bug for WebKit/Safari, Bug for WebKit/Safari.
ls4.45251314.149381
otherHTTP/3 protocol

Third version of the HTTP networking protocol which uses QUIC as transport protocol. Previously known as HTTP-over-QUIC, now standardized as HTTP/3.

Browser-specific notes:
#1: Can be enabled in Firefox via the network.http.http3.enabled pref in about:config.
#2: Can be enabled in Chrome(Chromium) via the --enable-quic & --quic-version=h3-23 command line arguments.
#3: Can be enabled in Chrome(Chromium) via the --enable-quic & --quic-version=h3-29 command line arguments.
#4: Requires macOS 11 Big Sur or later.
#5: Enabled by default for a portion of users.

Resources: Wikipedia article about HTTP/3.
other12514.0‒16.016.487871616.4148881
jsFeature Policy

This specification defines a mechanism that allows developers to selectively enable and disable use of various browser features and APIs. Feature Policy is deprecated and has been replaced with Permissions Policy and Document Policy.

Standard support includes the HTTP Feature-Policy header, allow attribute on iframes and the document.featurePolicy JS API.

Browser-specific notes:
#1: Older Chromium browsers did not support the JS API.
#2: Safari and Firefox only supports the allow attribute on iframes.
#3: Safari doesn't support a list of origins or a wildcard.
#4: At least partially supports Permissions Policy, the main replacement for this spec.

Resources: Feature Policy Kitchen Sink Demos, Introduction to Feature Policy, Firefox implementation ticket, Feature Policy Tester (Chrome DevTools Extension), featurepolicy.info (Feature-Policy Playground), List of known features.
unoff12511.3888811.111.17481
jscreateImageBitmap

Create image bitmap with support for resizing and adjusting quality.

Browser-specific notes:
#1: No support for options parameter, resizeWidth, resizeHeight, resizeQuality or SVGImageElement as Source Image.
#2: No support for resizeWidth, resizeHeight, resizeQuality or SVGImageElement as Source Image.
#3: No support for SVGImageElement as Source Image.
#4: No support for resizeWidth, resizeHeight and resizeQuality. See this bug.
#5: No support for createImageBitmap(source, options) interface. See this bug.
#6: Partial implementation. See this bug.
#7: No support for resizeQuality. See this bug.

Resources: self.createImageBitmap() - Web APIs | MDN.
ls125155979156.29881
cssCSS page-break properties

Properties to control the way elements are broken across (printed) pages.

Not all mobile browsers offer print support; support listed for these is based on browser engine capability.

Browser-specific notes:
#1: Supports the page-break-* alias from the CSS 2.1 specification, but not the break-* properties from the latest spec.
#2: Partial support is due to not supporting avoid for page-break-before & page-break-after (only page-break-inside).
#3: Treats the left and right values like always (as allowed by specification).

Resources: CSS Tricks article, Latest fragmentation specification (includes column & region breaks).
rec1253.210810810.1216581
jsService Workers

Method that enables applications to take advantage of persistent background processing, including hooks to enable bootstrapping of web applications while offline.

Details on partial support can be found on is ServiceWorker Ready?.

Browser-specific notes:
#1: Partial support can be enabled in Firefox with the dom.serviceWorkers.enabled flag.
#2: Available behind the "Enable service workers" flag.
#3: Disabled on Firefox ESR, but can be re-enabled with the dom.serviceWorkers.enabled flag.

Resources: HTML5Rocks article (introduction), MDN Web Docs - Service Workers, List of various resources.
cr12511.3451711.146980
otherAV1 video format

AV1 (AOMedia Video 1) is a royalty-free video format by the Alliance for Open Media, meant to succeed its predecessor VP9 and compete with the HEVC/H.265 format.

Edge has stopped supporting AV1 completely at some point prior to version 116 (additional information required).

Browser-specific notes:
#1: Can be enabled in Edge on Windows 10 or later by installing the AV1 Video Extension from the Microsoft Store.
#2: Firefox 65 only supported AV1 on Windows 64-bit.
#3: Firefox 66 only supported AV1 on Windows and macOS.
#4: Supported only on devices with hardware decoder, e.g. iPhone 15 Pro, M3 MacBook Pro, etc.

Resources: Wikipedia article, Sample video from Bitmovin, Sample video from Facebook, Firefox implementation bug, Safari implementation bug.
Parent feature: Video element.
other1253.2‒16.017701213.117126780
cssCSS Nesting

CSS nesting provides the ability to nest one style rule inside another, with the selector of the child rule relative to the selector of the parent rule. Similar behavior previously required a CSS pre-processor.

Browser-specific notes:
#1: Available behind the Experimental Web Platform features flag.
#2: Available behind the layout.css.nesting.enabled flag.
#3: Does not allow nesting of type (element) selectors without starting with a symbol, like the & nesting selector. Chromium issue.

Resources: Chrome support bug, Firefox support bug, Safari support bug, Blog post: CSS Nesting, specificity and you.
wd1253.2‒16.016.517.21091201203.116.517.22311777
otherCOLR/CPAL(v0) Font Formats

The COLR table adds support for multi-colored glyphs in a manner that integrates with the rasterizers of existing text engines. COLRv0 only supports pure colors, does not support gradients, transformations and various blending modes.

Browser-specific notes:
#1: Supported only on Safari for macOS High Sierra (10.13)+.
#2: Supported only on Windows 8.1+.
#3: On Windows pre 8.1 and macOS Sierra (10.12) and below falls back to using FreeType: Chromium bug 882844.
#4: COLRv0 support was disabled by a regression bug in Safari 17.0 and 17.1: Issue 262223.

Resources: Where can I use color fonts, DirectWrite's color fonts document, A tool to check whether the browser supports OpenType color formats, A variable color font, Make COLR/CPAL format fonts in Glyphs app.
Parent feature: @font-face Web fonts.
other12511.0‒16.01717.271110110141717.210.13275
jsWebCodecs API

API to provide more control over the encoding and decoding of audio, video, and images.

Browser-specific notes:
#1: Video-only support.

Resources: Explainer document, Video processing with WebCodecs, Firefox support bug, WebCodecs API on MDN, WebCodecs samples.
wd1253.2‒16.016.494943.116.417275
jsrequestIdleCallback

API allowing the execution of JavaScript to be queued to run in idle browser time, either at the end of a frame or when the user is inactive. Also covers support for cancelIdleCallback. The API has similarities with requestAnimationFrame.

Browser-specific notes:
#1: Can be enabled via the dom.requestIdleCallback.enabled flag.
#2: Can be enabled in Experimental (WebKit) Features.

Resources: MDN Web Docs - requestIdleCallback, Google Developers article, Shim.
wd12513.4477913.1‒17.0TP55575
css::selection CSS pseudo-element

The ::selection CSS pseudo-element applies rules to the portion of a document that has been highlighted (e.g., selected with the mouse or another pointing device) by the user.

Resources: ::selection test, MDN web docs.
wd4.43.2493.146273
html5Autofocus attribute

Allows a form field to be immediately focused on page load.

While not supported in iOS Safari, it does work in iOS WebViews.

Resources: Article on autofocus, MDN Web Docs - autofocus attribute.
ls33.251054473
css3CSS resize property

Method of allowing an element to be resized by the user, with options to limit to a given direction.

Browser-specific notes:
#1: Presto-based Opera 12.10+ currently only supports the resize property for textarea elements.

Resources: CSS Tricks info, On textarea resizing, CSS resize none on textarea is bad for UX.
rec1253.247945573
otherOgg Vorbis audio format

Vorbis is a free and open source audio format, most commonly used with the Ogg container.

Support refers to this format's use in the audio element, not other conditions.

Browser-specific notes:
#1: Supported on macOS Big Sur 11.3 or later.
#2: Supports the Vorbis audio codec, but not the Ogg container.

Resources: Wikipedia article.
Parent feature: Audio element.
other2.33.2‒17.017.441714.11743.571
css3CSS3 Cursors (original values)

CSS3 cursor values added in the 2004 spec, including none, context-menu, cell, vertical-text, alias, copy, no-drop, not-allowed, nesw-resize, nwse-resize, col-resize, row-resize and all-scroll.

Browser-specific notes:
#1: Partial support refers to no support for the alias, cell, copy, ew-resize, ns-resize, nesw-resize, nwse-resize or context-menu cursors.
#2: Partial support refers to not supporting 'none'.

Resources: MDN Web Docs - CSS cursor.
rec1253.251454470
css3CSS3 Cursors: zoom-in & zoom-out

Support for zoom-in, zoom-out values for the CSS3 cursor property.

Resources: MDN Web Docs - CSS cursor.
rec1253.23712942470
css3CSS grab & grabbing cursors

Support for the grab & grabbing values for the cursor property. Used to indicate that something can be grabbed (dragged to be moved).

Resources: MDN Web Docs - CSS cursor.
rec1253.268151142770
otherHTTP Live Streaming (HLS)

HTTP-based media streaming communications protocol.

HLS can be used with a JavaScript library in browsers that doesn't support it natively as long as they support Media Source Extensions.

Resources: Wikipedia article, Apple developer article.
unoff33.247964269
jsDo Not Track API

API to allow the browser's Do Not Track setting to be queried via navigator.doNotTrack. Due to lack of adoption the Do Not Track specification was deprecated in 2018.

Partial support refers to the doNotTrack field being misnamed, or being attached to an object other than navigator (e.g. window).

Browser-specific notes:
#1: IE 9 and 10 are vendor-prefixed as navigator.msDoNotTrack.
#2: IE 11 and Edge 16- use window.doNotTrack instead of navigator.doNotTrack.
#3: Browsers based on Gecko versions prior to 32 used 'yes' and 'no' rather than '1' and '0'.
#4: Between versions 6.1.1 and 9.1.3, Safari supported an ultimately-rejected amendment to the spec, using window.doNotTrack rather than navigator.doNotTrack.

Resources: MDN Web Docs - doNotTrack.
unoff4.412.2231712.143269
jsVibration API

Method to access the vibration mechanism of the hosting device.

Resources: MDN Web Docs - Vibration, Vibration API sample code & demo, Tuts+ article, Demo, Article and Usage Examples.
rec4.43.230793.141669
domAuxclick

The click event for non-primary buttons of input devices.

With introduction of this feature there will be no longer click event fired for non-primary buttons.

Browser-specific notes:
#1: As a compatibility measure, Firefox continues to fire the click event for document and window level event handlers.

Resources: MDN Web Docs - auxclick, Firefox implementation, WebKit bug, Original Proposal.
wd1253.255793.155369
css3CSS overflow-anchor (Scroll Anchoring)

Changes in DOM elements above the visible region of a scrolling box can result in the page moving while the user is in the middle of consuming the content.
By default, the value of  overflow-anchor is auto, it can mitigate this jarring user experience by keeping track of the position of an anchor node and adjusting the scroll offset accordingly.

Resources: Explainer, Google developers article, WebKit support bug.
wd1253.256793.156669
domMedia Capture from DOM Elements API

API to capture Real-Time video and audio from a DOM element, such as a <video>, <audio>, or <canvas> element via the captureStream method, in the form of a MediaStream.

Browser-specific notes:
#1: Does not support capture from <video>/<audio>.
#2: Capture from <video>/<audio> can be enabled via the Experimental Web Platform Features flag.

Resources: MDN Web Docs - capture from <canvas>, MDN Web Docs - capture from <video>/<audio>, Google Developers article.
Parent feature: getUserMedia/Stream API.
wd1253.26279118.24368
otherCOLR/CPAL(v1) Font Formats

COLRv1 is an improved version of COLRv0, this is also part of the OpenType specification. COLRv1 supports additional graphic capabilities. In addition to solid colors, gradient fills can be used, as well as more complex fills using other graphic operations, including affine transformations and various blending modes.

Browser-specific notes:
#1: Can be enabled with the "COLR v1 Fonts" feature flag under chrome://flags.
#2: Firefox actively supports this format and will consider implementing it in the future: https://mozilla.github.io/standards-positions/#font-colrv1.
#3: Can be enabled by setting gfx.font_rendering.colr_v1.enabled to true.

Resources: COLRv1 Color Gradient Vector Fonts in Chrome 98, COLRv1 version of Noto Emoji, A tool to check whether the browser supports OpenType color formats, Where can I use color fonts, Firefox support bug, WebKit position, First Batch of Color Fonts Arrives on Google Fonts.
Parent feature: @font-face Web fonts.
other1253.298983.11810768
jsWeb MIDI API

The Web MIDI API specification defines a means for web developers to enumerate, manipulate and access MIDI devices.

Resources: Polyfill, Firefox support bug, Test/demo page, WebKit support bug.
wd1253.243793.1410868
cssCSS content-visibility

Provides control over when elements are rendered, so rendering can be skipped for elements not yet in the user's viewport.

Browser-specific notes:
#1: Enabled in Firefox by setting the about:config preference layout.css.content-visibility.enabled to true.

Resources: content-visibility: the new CSS property that boosts your rendering performance, Firefox support bug, WebKit support bug.
wd1253.2‒17.01885853.1‒17.0181412568
css3text-decoration styling

Method of defining the type, style and color of lines in the text-decoration property. These can be defined as shorthand (e.g. text-decoration: line-through dashed blue) or as single properties (e.g. text-decoration-color: blue).

All browsers support the CSS2 version of text-decoration, which matches only the text-decoration-line values (underline, etc.).

Browser-specific notes:
#1: Enabled in Chrome through the "experimental Web Platform features" flag in chrome://flags.
#2: Partial support refers to not supporting the text-decoration-style property.
#3: Safari 8+ supports -webkit-text-decoration-skip with values none and skip (other values behave like none or skip).
#4: Partial support refers to not supporting the text-decoration-skip property.
#5: Partial support refers to text-decoration-skip only supporting value objects and ink.

Resources: MDN Web Docs - text-decoration-style, MDN Web Docs - text-decoration-color, MDN Web Docs - text-decoration-line, MDN Web Docs - text-decoration-skip, Firefox implementation bug.
cr1258577912.17.23667
jsWebTransport

Protocol framework to send and receive data from servers using HTTP3. Similar to WebSockets but with support for multiple streams, unidirectional streams, out-of-order delivery, and reliable as well as unreliable transport.

Resources: web.dev article, Firefox support bug, Explainer with examples, WebKit position on WebTransport.
wd1253.297983.11811467
html5Number input type

Form field type for numbers.

Browser-specific notes:
#1: Does not support increment/decrement, either via buttons in UI or via arrow up & down keys.
#2: UI widget does not take the "step", "min" or "max" attributes into account.
#3: Firefox doesn't support autocomplete content via datalist elements.
#4: Does not include increment/decrement buttons, but does support increment/decrement via arrow up & down keys.

Resources: Polyfill, has.js test, WebPlatform Docs, Poor browser support for localized decimal marks, commas.
ls43.2679542966
html5Spellcheck attribute

Attribute for input/textarea fields to enable/disable the browser's spellchecker.

The partial support in mobile browsers results from their OS generally having built-in spell checking instead of using the wavy underline to indicate misspelled words. spellcheck="false" does not seem to have any effect in these browsers.

Browsers have different behavior in how they deal with spellchecking in combination with the lang attribute. Generally spelling is based on the browser's language, not the language of the document.

Resources: MDN Web Docs - Controlling spell checking.
ls2.13.29105.14266
html5HTML Media Capture

Facilitates user access to a device's media capture mechanism, such as a camera, or microphone, from within a file upload control.

Browser-specific notes:
#1: Does not support the capture attribute used to force capture straight from the device's camera or microphone. Also note that default video dimensions are 480x320 (4:3).
#2: Android 2.2-2.3 do not support the capture attribute.
#3: Supports a "capture" button for any `<input type="file"> field, regardless of whether the capture attribute is used.

Resources: Correct Syntax for HTML Media Capture, Programming the Mobile Web: File upload compatibility table, HTML Media Capture Test Bench.
rec3645.53.14265
cssCSS text-indent

The text-indent property applies indentation to lines of inline content in a block.

Browser-specific notes:
#1: Partial support refers to supporting a <length> value, but not the each-line or hanging keywords.
#2: Support for each-line & hanging is available behind the Experimental Web Platform features flag.

Resources: MDN Web Docs - CSS text-indent, Firefox support bug, WebKit support bug, Article on using text-indent for image replacement.
cr1253.216387916512165
domImageCapture API

The Image Capture API provides access to the Video Camera for taking photos while configuring picture-specific settings such as e.g. zoom or auto focus metering area.

Firefox supports the takePhoto() method only (when flag is enabled).

Browser-specific notes:
#1: Can be enabled via the about:config entry dom.imagecapture.enabled.
#2: Can be enabled via the Experimental Web Platform Features flag.

Resources: Minimal code pen, Extended demo, Firefox tracking bug.
Parent feature: getUserMedia/Stream API.
wd1253.259793.153565
cssCSS Painting API

Allows programmatic generation of images used by CSS.

Browser-specific notes:
#1: Can be enabled using the Develop > Experimental Features menu.

Resources: Google CSS Paint API Introduction, Is Houdini Ready Yet?.
cr1253.2657912.19.2265
css3CSS3 Multiple column layout

Method of flowing information in multiple columns.

Browser-specific notes:
#1: Does not support the properties break-after, break-before, and break-inside. WebKit- and Blink-based browsers have equivalent support with the non-standard -webkit-column-break-* properties, but only for the values auto and always. Firefox does not support the break-* properties but supports the now-obsolete page-break-* properties in the paging (printing) context.
#2: Does not support the column-fill property.
#3: Does not support the values avoid (in the column context), avoid-column, and avoid-page for the properties break-after, break-before, and break-inside; does not support the value column for the properties break-after and break-before.
#4: Does not support the values avoid (in the column context) for the properties break-after, break-before, and break-inside; does not support the values avoid-column, avoid-page, and column for the properties break-before and break-after.

Resources: Dev.Opera article, Introduction page, WebPlatform Docs, Polyfill, Chrome platform status for CSS column-fill.
cr1251050791059264
jsSynchronous Clipboard API

API to provide copy, cut and paste events as well as provide access to the OS clipboard.

Internet Explorer will display a security prompt for access to the OS clipboard.

Chrome 42+, Opera 29+ and Firefox 41+ support clipboard reading/writing only when part of a user action (click, keydown, etc).

Browser-specific notes:
#1: Only supports Text and URL data types and uses a non-standard method of interacting with the clipboard.
#2: Only fires copy event on a valid selection and only cut and paste in focused editable fields.
#3: Only supports OS clipboard reading/writing via shortcut keys, not through document.execCommand().
#4: Only supports paste event (on focused editable field).
#5: Does not support the ClipboardEvent constructor.
#6: Supports cut & copy events without a focused editable field, but not paste (presumably for security reasons).
#7: Supports cut & copy events without a focused editable field, but does not fire paste with document.execCommand('paste').

Resources: MDN Web Docs - ClipboardEvent, Guide on cross-platform clipboard access.
wd125125879127.24164
jsBattery Status API

Method to provide information about the battery status of the hosting device.

Firefox 52+ removed access to this API due to privacy concerns..

Browser-specific notes:
#1: Partial support refers to support for the older specification's navigator.battery rather than navigator.getBattery() to access the BatteryManager.

Resources: MDN Web Docs - battery status, Simple demo.
cr1253.238793.145264
domClient Hints: DPR, Width, Viewport-Width

DPR, Width, and Viewport-Width hints enable proactive content negotiation between client and server, enabling automated delivery of optimized assets - e.g. auto-negotiating image DPR resolution.

Resources: Automating resource selection with Client Hints, Mozilla Bug 935216 - Implement Client-Hints HTTP header, WebKit Bug 145380 - Add Content-DPR header support.
other1253.246793.15264
jsBackground Sync API

Provides one-off and periodic synchronization for Service Workers with an onsync event.

Resources: Firefox support bug, SyncManager on MDN Web Docs, Google Developers blog: Introducing Background Sync.
unoff1253.249793.15212864
jsWeb Bluetooth

Allows web sites to communicate over GATT with nearby user-selected Bluetooth devices in a secure and privacy-preserving way.

Browser-specific notes:
#1: Available by enabling the "Web Bluetooth" experimental flag in about:flags. Currently support varies by OS.
#2: Only in Opera Mobile.
#3: Available in Origin Trials for Chrome OS, Android M, and Mac.
#4: Currently support varies by OS.

Resources: Intro, Samples, Demos, Implementation Status, Mozilla Specification Positions: Harmful.
unoff1253.256793.16.2264
jsAsynchronous Clipboard API

A modern, asynchronous Clipboard API based on Promises.

Browser-specific notes:
#1: Partial support in Firefox refers to only supporting the "clipboard-write" permission and the "writeText" method.
#2: Supports writing to clipboard but not reading.

Resources: MDN Web Docs - Clipboard API, W3C Async Clipboard Explainer, Unlocking Clipboard Access, Firefox implementation bug.
wd12514627913.19.26364
otherSigned HTTP Exchanges (SXG)

Part of the Web Packaging spec, Signed HTTP Exchanges allow a different origin server to provide a resource, and this will be treated as if it came from the original server. This can be used with AMP CDNs, for example, to allow the original URL to be displayed in the URL bar.

Note this requires the page to be delivered signed by a certificate with the CanSignHttpExchanges extension.

Browser-specific notes:
#1: Partial support behind a feature flag (chrome://flags/#enable-signed-http-exchange).

Resources: Chrome platform status - Shipped, Microsoft Edge Platform Status - Supported, Signed HTTP Exchanges on Google's Web Development site, Developer Preview of better AMP URLs in Google Search, Signed-Exchange: Solving the AMP URLs Display Problem, GitHub home page for Web Packaging, Mozilla's Position about Signed HTTP Exchanges (harmful).
other1253.273793.111.1263
domIntersectionObserver V2

Iteration on the original API that also reports if the element is covered by another element or has filters applied to it. Useful for blocking clickjacking attempts or tracking ad exposure.

Resources: Google Web Docs - Intersection Observer V2, Request for Mozilla Position on IntersectionObserver V2, Safari support bug.
Parent feature: IntersectionObserver.
unoff1253.274793.111.1263
domTrusted Types for DOM manipulation

An API that forces developers to be very explicit about their use of powerful DOM-injection APIs. Can greatly improve security against XSS attacks.

Resources: Web.dev article on using trusted types, Firefox position: non-harmful.
unoff1253.283833.113263
otherHEVC/H.265 video format

The High Efficiency Video Coding (HEVC) compression standard is a video compression format intended to succeed H.264. It is hard for browsers to universally support HEVC because it is complex and expensive to license. HEVC competes with AV1 which has similar compression quality and provides a free license.

Browser-specific notes:
#1: Supported only for devices with hardware support.
#2: Reported to work in certain Android devices with hardware support.
#3: Supported only on macOS High Sierra or later.
#4: Supported for all devices on macOS (>= Big Sur 11.0) and Android (>= 5.0) if Edge >= 107, for devices with hardware support on Windows (>= Windows 10 1709) when HEVC video extensions from the Microsoft Store is installed.
#5: Supported for all devices on macOS (>= Big Sur 11.0) and Android (>= 5.0), for devices with hardware support on Windows (>= Windows 8), and for devices with hardware support powered by VAAPI on Linux and ChromeOS.
#6: Supported for devices with hardware support (the range is the same as Edge) on Windows in Nightly only. 10-bit or higher colors are not supported.
#7: Supported for devices with hardware support (the range is the same as Edge) on Windows only. Enabled by default in Nightly and can be enabled via the media.wmf.hevc.enabled pref in about:config. 10-bit or higher colors are not supported.

Resources: Firefox support bug (WONTFIX), Wikipedia article, Chrome support bug (WontFix), Firefox support via OS API bug.
Parent feature: Video element.
other1251110779132112163
otherzstd (Zstandard) content-encoding

Data compression method providing faster page loading while using less CPU power on the server.

Browser-specific notes:
#1: Enabled through the enable-zstd-content-encoding feature flag in chrome://flags.

Resources: Official Zstandard site, Wikipedia article, Support test, Firefox support bug, WebKit position on zstd.
other1253.241231233.1412663
jsWeb Notifications

Method of alerting the user outside of a web page by displaying notifications (that do not require interaction by the user).

Browser-specific notes:
#1: Supports notifications via the Push API but not the Web Notifications API.
#2: Chrome for Android requires the call to be made with a service worker registration.
#3: Requires website to first be added to the Home Screen.

Resources: HTML5 Rocks tutorial, Chromium API, Add-on, MDN Web Docs - Notification, SitePoint article, Demo, Plug-in for support in IE.
ls4.43.2‒16.016.42214652262
jsGamepad API

API to support input from USB gamepad controllers through JavaScript.

Resources: Controller demo, MDN Web Docs - Gamepad, HTML5Rocks article, Detailed tutorial.
wd2.110.3251210.142962
jsSpeech Synthesis API

A web API for controlling a text-to-speech output.

Samsung Internet for GearVR: In Development, release based on Chromium m53 due Q1/2017.

Browser-specific notes:
#1: Can be enabled in Firefox using the media.webspeech.synth.enabled about:config flag.
#2: Speech Synthesis in Chrome since version 55 stops playback after about 15 seconds on Windows 7 & 10, and Ubuntu 14.04, possibly other platforms.

Resources: SitePoint article, Demo, Advanced demo and resource, MDN article, Google Developers article.
Parent feature: .
unoff2.175579754962
cssView Transitions API (single-document)

Provides a mechanism for easily creating animated transitions between different DOM states, while also updating the DOM contents in a single step. This API is specific to single-document transitions, support for same-origin cross-document transitions is being planned.

Resources: Explainer document, View Transitions API on MDN, Chrome Developers documentation, Firefox support bug.
wd1253.2‒17.01841111113.1‒17.01823262
cssScoped Styles: the @scope rule

Allows CSS rules to be scoped to part of the document, with upper and lower limits described by selectors.

This implementation replaces an older concept of scoping CSS rules.

Browser-specific notes:
#1: Enabled in Chrome through the "experimental Web Platform features" flag in chrome://flags.

Resources: Explainer, Firefox support bug, WebKit position: support, An introduction to @scope in CSS.
wd1253.2‒17.017.41041181183.1‒17.017.425262
domMediaRecorder API

The MediaRecorder API (MediaStream Recording) aims to provide a really simple mechanism by which developers can record media streams from the user's input devices and instantly use them in web apps, rather than having to perform manual encoding operations on raw PCM data, etc.

Browser-specific notes:
#1: Can be enabled via the experimental Web Platform features flag. Does not support audio recording, only video.
#2: Can be enabled using the Develop > Experimental Features menu.
#3: Can be enabled using the Advanced > Experimental Features menu.

Resources: MDN Web Docs - MediaRecorder.
Parent feature: getUserMedia/Stream API.
wd2.114.5497914.152961
jsGyroscope

Defines a concrete sensor interface to monitor the rate of rotation around the device’s local three primary axes.

Browser-specific notes:
#1: Available by enabling the "Generic Sensor" experimental flag in about:flags.

Resources: Demo, Article.
cr1253.267793.14261
jsAccelerometer

Defines Accelerometer, LinearAccelerationSensor and GravitySensor interfaces for obtaining information about acceleration applied to the X, Y and Z axis of a device that hosts the sensor.

Browser-specific notes:
#1: Available by enabling the "Generic Sensor" experimental flag in about:flags.

Resources: Demo, Article.
cr1253.267793.14261
jsOrientation Sensor

Defines a base orientation sensor interface and concrete sensor subclasses to monitor the device’s physical orientation in relation to a stationary three dimensional Cartesian coordinate system.

Browser-specific notes:
#1: Available by enabling the "Generic Sensor" experimental flag in about:flags.

Resources: Demo, Article.
cr1253.267793.14261
html5Multiple file selection

Allows users to select multiple files in the file picker.

Browser-specific notes:
#1: Not supported on Android 4.x and below, presumably an OS limitation. Only seems to work in Android 5.x for the Chrome browser.

Resources: Chrome bug (for Android), Article.
ls2.16510453.661
jsShared Array Buffer

Type of ArrayBuffer that can be shared across Workers.

Browser-specific notes:
#1: Has support, but was disabled across browsers in January 2018 due to Spectre & Meltdown vulnerabilities.
#2: Enabled by default in Nightly, but not in Beta/Developer/Release.
#3: Requires cross-origin isolation by having Cross-Origin-Embedder-Policy (COEP) and Cross-Origin-Opener-Policy (COOP) headers set. Mozilla Hacks article for context.

Resources: MDN article, Mozilla Hacks article on safely reviving shared memory.
other2.110.315.2919115.2157960
jsPermissions API

High-level JavaScript API for checking and requesting permissions.

Resources: Permission API samples and examples, Extended "polyfill" version of permission API.
wd2.13.21643791644660
domKeyboardEvent.code

A KeyboardEvent property representing the physical key that was pressed, ignoring the keyboard layout and ignoring whether any modifier keys were active.

"Some key events, or their values, might be suppressed by the IME in use". On mobile (virtual keyboard), all keys are reported as empty ("").

Browser-specific notes:
#1: Enabled in Chrome & Opera through the "experimental Web Platform features" flag.

Resources: MDN Web Docs - code, Chrome tracking bug, WebKit feature request bug.
wd2.110.3487910.153860
html5tabindex global attribute

Specifies the focusability of the element and in what order (if any) it should become focused (relative to other elements) when "tabbing" through the document.

Mac OS X "Full Keyboard Access" refers to setting Keyboard→Shortcuts→Full Keyboard Access to "All controls" in the System Preferences.

"Unknown" support for mobile browsers is due to lacking a method of tabbing through fields.

Browser-specific notes:
#1: On Mac OS X, unless Full Keyboard Access is enabled, <a> elements are not keyboard-focusable, even if they have tabindex="0".
#2: <a> elements are never keyboard-focusable, even if they have tabindex="0". Unless Full Keyboard Access is enabled, then <button>s, radio buttons, and checkboxes are also not keyboard-focusable, even if they have tabindex="0".
#3: Has "previous" and "next" virtual keyboard buttons that follow tabindex order.

Resources: MDN Web Docs - tabindex attribute.
ls2.13.21575.14460
html5Printing Events

Window fires beforeprint and afterprint events so the printed document can be annotated.

Due to its wider support, consider using window.matchMedia('print') where possible.

Resources: MDN Web Docs - Detecting print requests, Chrome support bug, Safari support bug.
ls2.113636135659
cssCSS display: contents

display: contents causes an element's children to appear as if they were direct children of the element's parent, ignoring the element itself. This can be useful when a wrapper element should be ignored when using CSS grid or similar layout techniques.

Browser-specific notes:
#1: Partial support refers to severe implementation bugs that renders content inaccessible for many element types.
#2: Buttons are not accessible with display: contents applied. See issues for Chromium, Firefox, and WebKit.
#3: HTML tables and nodes with ARIA roles directory, grid, treegrid, table, row, gridcell, cell, columnheader, tree, and treeitem are not accessible with display: contents applied. See WebKit bugs 239478, 239479, 239478, and 257458.

Resources: Vanishing boxes with display contents.
cr12513.4161789891617156259
otherBuilt-in PDF viewer

Support for a PDF viewer that is part of the browser, rather than requiring a PDF file to be opened in an external application.

When displaying PDFs inline rather than separately, iOS Safari will only display the first page of the document.

Browser-specific notes:
#1: Not supported in IE11 before Windows 10.

Resources: PDFObject - JavaScript utility to embed PDF documents in HTML.
other2.13.21515441959
jsPayment Request API

Payment Request is a new API for the open web that makes checkout flows easier, faster and consistent on shopping sites.

Apple provides an equivalent proprietary API called Apple Pay JS. Google provides a PaymentRequest wrapper for Apple Pay JS.

Browser-specific notes:
#1: Can be enabled via the "Experimental Web Platform features" flag.
#2: Can be enabled via the "Experimental Web Payments API" flag.
#3: Apple's proprietary implementation (see above).
#4: Can be enabled via the "Web Payments API" flag.
#5: Unlike Desktop Chrome, support has been in Chrome for Android since version 53.
#6: Can be enabled via the dom.payments.request.enabled flag in "about:config" flag since 55.
#7: Missing support for PaymentResponse.prototype.retry() method.
#8: Disabled by default in Nightly. To enable, go to "about:config" and set "dom.payments.request.enabled" to true, and "dom.payments.request.supportedRegions" to "US,CA".

Resources: Spec discussion, Bringing easy and fast checkout with Payment Request API, Payment Request API Integration Guide, MDN Web Docs - Payment Request API, Demo, Simpler Demos and Codes.
rec2.112.2787912.1126658
jsShared Web Workers

Method of allowing multiple scripts to communicate with a single web worker.

Resources: Sitepoint article, Blog post.
Parent feature: Web Workers.
ls2.17164791652958
jsPasskeys

Passkeys, also known as Multi-device FIDO Credentials, provide users with an alternative to passwords that is much easier to use and far more secure.

Support for various passkey features also depends on the device and OS used. See the passkeys.dev site for details.

Browsers without Passkey support may still allow authentication at passkey logins via other types of security keys as long as they support WebAuthn.

Resources: Passkeys at apple.com, Firefox support bug, Google article on Passkeys for developers, FIDO alliance article on Passkeys.
other2.13.2161081083.116.12112257
jsPicture-in-Picture

Allows websites to create a floating video window that is always on top of other windows so that users may continue consuming media while they interact with other sites or applications on their device.

Browser-specific notes:
#1: Partial support in Firefox refers to the fact that Mozilla provides an equivalent proprietary feature, which allows users to use Picture-in-Picture mode for all playing videos.
#2: Partial support in Firefox 71 refers to being enabled by default only for Windows users.
#3: Partial support in Firefox 68–70 refers to being enabled by default in Firefox pre-release channels for Windows users only.
#4: Can be enabled in Firefox via the media.videocontrols.picture-in-picture.enabled pref in about:config.
#5: Can be enabled in Chrome via the #enable-picture-in-picture & #enable-surfaces-for-videos flags in chrome://flags.
#6: Partial support in Safari refers to the fact that Apple provides an equivalent proprietary API for Picture-in-Picture, which is only supported by macOS 10.12 Sierra & iOS 9 and newer.
#7: Partial support in Opera refers to the fact that Opera provides an equivalent proprietary feature named Video Pop Out, which allows users to use Picture-in-Picture mode for all playing videos.
#8: Automatically plays fullscreen videos in Picture-in-Picture mode after user hits Home Screen button. Only Android 8.0 Oreo and newer provide appropriate API for Picture-in-Picture.

Resources: Sample video, Safari equivalent API, Opera equivalent Video Pop Out feature, Implementation Status, Firefox implementation bug.
wd2.114707913.147257
cssCSS Anchor Positioning

Allows placing elements anywhere on the page relative to an "anchor element", without regard to the layout of other elements besides their containing block.

Browser-specific notes:
#1: Can be enabled via the #enable-experimental-web-platform-features flag in chrome://flags.

Resources: Blog post on usage, Polyfill, Firefox support bug, Blog post, WebKit position on Anchor Positioning.
wd1253.24117‒1241251253.14256
jsScreen Orientation

Provides the ability to read the screen orientation state, to be informed when this state changes, and to be able to lock the screen orientation to a specific state.

Partial support refers to an older version of the draft specification, and the spec has undergone significant changes since, for example renaming the screen.lockOrientation method to screen.orientation.lock.

Browser-specific notes:
#1: Not supported on Windows 7.

Resources: Demo, MDN Web Docs - Screen Orientation, SitePoint article.
wd2.13.2‒16.016.438793.116.454455
cssBackground-clip: text

Clipping a background image to the foreground text.

Firefox and legacy Edge also support this property with the -webkit- prefix.

Browser-specific notes:
#1: Doesn't work with a stacking context(e.g. position: relative), see Chromium bug.
#2: Doesn't work with flex/grid container, see WebKit bug.

Resources: [css-backgrounds] Standardize 'background-clip: text', CSS Backgrounds and Borders Module Level 4, MDN Web Docs - background-clip.
Parent feature: CSS3 Background-image options.
unoff1251415.5412012015.5254953
svgSVG effects for HTML

Method of using SVG transforms, filters, etc on HTML elements using either CSS or the foreignObject element.

Partial support refers to lack of filter support or buggy result from effects. A CSS Filter Effects specification is in the works that would replace this method.

Browser-specific notes:
#1: IE11 and below do not support <foreignObject>.
#2: IE and Edge do not support applying SVG filter effects to HTML elements using CSS. Bug Report.

Resources: MDN Web Docs - Other content in SVG, MDN Web Docs - Applying SVG effects, Filter Effects draft.
rec4.43.2479443.552
html5Ruby annotation

Method of adding pronunciation or other annotations using ruby elements (primarily used in East Asian typography).

Browsers without native support can still simulate support using CSS. Partial support refers to only supporting basic ruby, may still be missing writing-mode, Complex ruby and CSS3 Ruby.

Browser-specific notes:
#1: IE9+ supports properties of an older version of the CSS Ruby specification.

Resources: HTML5 Doctor article, WebPlatform Docs, CSS specification.
ls35512543852
css3CSS box-decoration-break

Controls whether the box's margins, borders, padding, and other decorations wrap the broken edges of the box fragments (when the box is split by a break (page/column/region/line).

Browser-specific notes:
#1: Partial support refers to working for inline elements but not across column or page breaks.

Resources: MDN Web Docs - CSS box-decoration-break, Demo of effect on box border, Chromium bug to unprefix `-webkit-box-decoration-break`.
cr4.4722796.143252
css3CSS clip-path property (for HTML)

Method of defining the visible region of an HTML element using SVG or a shape definition.

Support refers to the clip-path CSS property on HTML elements specifically. Support for clip-path in SVG is supported in all browsers with basic SVG support.

Browser-specific notes:
#1: Partial support refers to only supporting the url() syntax.
#2: Partial support refers to supporting shapes and the url(#foo) syntax for inline SVG, but not shapes in external SVGs.
#3: Supports shapes behind the layout.css.clip-path-shapes.enabled flag.
#4: Partial support can be enabled on MS Edge with the Enable CSS Clip-Path Flag.
#5: While Edge doesn't seem to recognize any <basic-shape> function, it does allow you to use polygon() in HTML and SVG.

Resources: CSS Tricks article, Codepen Example Clipping an Image with a Polygon, Visual test cases, Chromium bug for shapes in external SVGs, WebKit bug for shapes in external SVGs.
Parent feature: CSS Masks.
cr12513557913.16.25452
domNetwork Information API

The Network Information API enables web applications to access information about the network connection in use by the device. Accessed via navigator.connection.

Browser-specific notes:
#1: Supports only the navigator.connection.type value which doesn't match the latest spec. see details.
#2: Only supports the type value.
#3: Only supports the type and downlinkMax values.
#4: Only supports the downlink, effectiveType & rtt values.

Resources: (NetInfo) Capability reporting with ServiceWorker.
unoff1253.261793.18.2251
cssCSS scrollbar styling

Methods of styling scrollbars' color and width.

Browser-specific notes:
#1: Only supports styling scrollbar colors through proprietary prefixed properties, no other properties to define the scrollbar's appearance. (not on standards track).
#2: Supports scrollbar styling via CSS pseudo-properties. (not on standards track).
#3: Scrollbar styling doesn’t work in WKWebView.
#4: Can be enabled by setting the layout.css.scrollbar-colors.enabled and layout.css.scrollbar-width.enabled flags to true.
#5: Firefox 63 supported scrollbar-face-color and scrollbar-track-color separate properties (now dropped from the spec) instead of unified scrollbar-color property.

Resources: Firefox support bug, Stackoverflow article discussing cross-browser support, Tutorial for IE & WebKit/Blink browsers, "perfect-scrollbar" - Minimal custom scrollbar plugin, jQuery custom content scroller, Webkit blog post describing their non-standard support.
cr2.31441211215.146450
securityX-Frame-Options HTTP header

An HTTP header which indicates whether the browser should allow the webpage to be displayed in a frame within another webpage. Used as a defense against clickjacking attacks.

Partial support refers to not supporting the ALLOW-FROM option.
The X-Frame-Options header has been obsoleted by the frame-ancestors directive from Content Security Policy Level 2.

Resources: X-Frame-Options Compatibility Test, MDN Web Docs - X-Frame-Options, OWASP Clickjacking Defense Cheat Sheet, Combating ClickJacking With X-Frame-Options - IEInternals, IE8 Security Part VII: ClickJacking Defenses - IEBlog.
other4726795.147049
securityDNSSEC and DANE

Method of validating a DNS response against a trusted root server. Mitigates various attacks that could reroute a user to a fake site while showing the real URL for the original site.

Browsers have generally decided to not implement DNSSEC validation because the added complexity outweighs the improvements to the browser. DNSSEC is still useful as it is widely used to protect delivery of records between DNS servers, only failing to protect the delivery from the last DNS server to the browser.

Certificate transparency is widely used and tries to provide the same security as DNSSEC but by very different means.

Browser-specific notes:
#1: Does not support DNSSEC but still benefits greatly from it, as most of the security improvements are gained in the DNS-network and not in the browser.
#2: Early versions of Chrome experimented with browser-level validation.

Resources: Wikipedia - DNSSEC, Chrome implementation bug, Firefox implementation bug.
other2.13.2315.53.14249
otherWAI-ARIA Accessibility features

Method of providing ways for people with disabilities to use dynamic web content and web applications.

Support for ARIA is rather complex and currently is not fully supported in any browser. For detailed information on partial support see the ARIA 1.0 Implementation Report.

Resources: Wikipedia, HTML5/WAI-ARIA information, a11ysupport.io - Accessibility Support data for various HTML, ARIA, CSS, and SVG features, WAI-ARIA Overview, Links to various test results, A List Apart - The Accessibility of WAI-ARIA.
rec4.43.24844249
domMutation events

Deprecated mechanism for listening to changes made to the DOM, replaced by Mutation Observers.

See also support for Mutation Observer, which replaces mutation events and does not have the same performance drawbacks.

Browser-specific notes:
#1: Does not support DOMAttrModified.
#2: Does not support DOMNodeInsertedIntoDocument & DOMNodeRemovedFromDocument.

Resources: MDN Web Docs - Mutation events.
wd2.34.2157944649
domDOMMatrix

The DOMMatrix interface represents 4x4 matrices, suitable for 2D and 3D operations. Supersedes the WebKitCSSMatrix and SVGMatrix interfaces.

Browser-specific notes:
#1: Only supports the WebKitCSSMatrix version of the interface, not DOMMatrix.
#2: WebKitCSSMatrix#skewX, WebKitCSSMatrix#skewY are not supported.
#3: Only supports the DOMMatrix version of the interface, not WebKitCSSMatrix (support required by spec for legacy reasons).
#4: Only replaces WebkitCSSMatrix and not SVGMatrix.
#5: Does not support fromMatrix(), fromFloat32Array(), fromFloat64Array(), and toJSON() methods.

Resources: WebKitCSSMatrix API Reference, WebKitCSSMatrix in Compatibility Standard, MDN Web Docs - DOMMatrix, Chrome implementation bug.
cr1251161791146949
domMedia Fragments

Allows only part of a resource to be shown, based on the fragment identifier in the URL. Currently support is primarily limited to video track ranges.

Browser-specific notes:
#1: Only appears to support the #t=n,n control for selecting a range of video, and possibly track=(name) & id=(name) (not yet tested).

Resources: Media fragments on MDN.
rec4.48187966.23449
cssFullscreen API

API for allowing content (like a video or canvas element) to take up the entire screen.

Browser-specific notes:
#1: Partial support refers to supporting an earlier draft of the spec.
#2: Partial support refers to not supporting ::backdrop, and supporting the old :full-screen syntax rather than the standard :fullscreen.
#3: Partial support refers to not returning a Promise, as specified in the latest version of the spec.
#4: Unprefixed support is available behind the full-screen-api.unprefix.enabled flag.
#5: Partial support refers to supporting only iPad, not iPhone. Shows an overlay button which can not be disabled.

Resources: MDN Web Docs - Using Full Screen, Mozilla hacks article, WebPlatform Docs.
ls2.112717915.416.410.16448
html5accept attribute for file input

Allows a filter to be defined for what type of files a user may pick with from an <input type="file"> dialog.

Not supported means any file can be picked as if the accept attribute was not set, unless otherwise noted.

On Windows, files that do not apply are hidden. On OSX they are grayed out and disabled.

Browser-specific notes:
#1: Supports the type format (e.g. image/*) but not the extension format (e.g. .png).
#2: Offers appropriate file locations/input based on format type, but does not prevent other files from being selected.
#3: Does not allow any files to be picked at all.
#4: Supports the type format but does not allow any file to be picked when using the extension format.

Resources: Demo & information.
ls4.48267911.143747
jsDeviceOrientation & DeviceMotion events

API for detecting orientation and motion events from the device running the browser.

Partial support refers to the lack of compassneedscalibration event. Partial support also refers to the lack of devicemotion event support for Chrome 30- and Opera. Opera Mobile 14 lost the ondevicemotion event support. Firefox 3.6, 4 and 5 support the non-standard MozOrientation event.

Browser-specific notes:
#1: compassneedscalibration supported in IE11 only for compatible devices with Windows 8.1+.

Resources: HTML5 Rocks tutorial, has.js test, DeviceOrientation implementation prototype for IE10, Demo.
cr34.27793.14647
jsWeb Share API

A way to allow websites to invoke the native sharing capabilities of the host platform.

Browser-specific notes:
#1: Implemented old Web Intents.
#2: Android intent:// URLs can be used instead.
#3: Only supported on Windows.
#4: Only supported on Windows and Chrome OS.
#5: Did not support share click that would trigger a fetch call.

Resources: Chrome post, Hospodarets - Web Share API brings the native sharing capabilities to the browser, Phil Nash - The Web Share API, Mozilla bug #1312422: Consider experimental support for Web Share API, How to Use the Web Share API to Trigger the Native Dialog to Share Content & Pull Quotes, Web.dev - Share like a native app with the Web Share API, Firefox support bug, W3C Demo, Chromium support bug for macOS, Web Share API on MDN.
cr4.4.3148995148.2246
jsFile System Access API

API for manipulating files in the device's local file system (not in a sandbox).

Browser-specific notes:
#1: Can be enabled in desktop Chromium browsers with the #native-file-system-api flag.
#2: Desktop Chromium browsers currently support basic functionality and will be adding more of the API in the future.
#3: Only supports the Origin Private File System (OPFS) part.

Resources: Explainer, Web.dev blog post, Firefox position: harmful, Chrome blog post.
unoff1253.215.2868615.2411146
cssCSS Initial Letter

Method of creating an enlarged cap, including a drop or raised cap, in a robust way.

Browser-specific notes:
#1: Safari implementation is incomplete. Does not allow applying web fonts to the initial letter.
#2: Partial support refers to only supporting the initial-letter property (not initial-letter-align or initial-letter-wrap).

Resources: Firefox Implementation Ticket, MDN Web Docs - CSS initial-letter, Blog post on Envato Tuts+, "Better CSS Drop Caps With initial-letter", Demos at Jen Simmons Labs, WebKit bug to unprefix -webkit-initial-letter.
wd125941101109.121246
css3CSS Counter Styles

The @counter-style CSS at-rule allows custom counter styles to be defined. A @counter-style rule defines how to convert a counter value into a string representation.

Browser-specific notes:
#1: Partial support refers to lacking support for image symbols: Firefox bug, Chrome bug.

Resources: MDN Web Docs - CSS counter style, CSS @counter-style Demo, WebKit bug on support for @counter-style.
cr1253.2‒16.01791913.117163346
jsPush API

API to allow messages to be pushed from a server to a browser, even when the site isn't focused or even open in the browser.

Browser-specific notes:
#1: Partial support refers to not supporting PushEvent.data and PushMessageData.
#2: Requires full browser to be running to receive messages.
#3: Safari 7.0 - 16.0 supported a custom implementation which remains available in later versions, see Safari Push Notifications and WWDC video.
#4: Disabled on Firefox ESR, but can be re-enabled with the dom.serviceWorkers.enabled and dom.push.enabled flags.
#5: Only available on macOS 13 Ventura or later and only in Safari itself, not WKWebView nor SFSafariViewController.
#6: Requires website to first be added to the Home Screen. Delivered silently, meaning no sound, vibration, haptics or screen wake.

Resources: MDN Web Docs - Push API, Google Developers article.
wd2.13.2‒16.016.45079716.146144
domtheme-color Meta Tag

Meta tag to define a suggested color that browsers should use to customize the display of the page or of the surrounding user interface. The meta tag overrides any theme-color set in the web app manifest.

Vivaldi browser (Chromium based) supports this feature in both mobile and desktop version. Also supported in the (now discontinued) Firefox OS.

Browser-specific notes:
#1: Desktop Chrome only uses the color on installed progressive web apps.
#2: Desktop Chrome 39-72 claimed to have support, but did not actually use the color anywhere. Chrome for Android did use the color in the toolbar.
#3: Chrome for Android does not use the color on devices with native dark-mode enabled before Chrome 92 unless it's an installed progressive web app or a trusted web activity.

Resources: Firefox for Android implementation bug, Google Developers article.
ls2.115735.5156.2241
otherSDCH Accept-Encoding/Content-Encodingother1253.2595.53.15239
jsasm.js

An extraordinarily optimizable, low-level subset of JavaScript, intended to be a compile target from languages like C++.

asm.js is mostly rendered obsolete with the introduction of WebAssembly and is therefore no longer recommended.

Browser-specific notes:
#1: Chromium does not support ahead-of-time compilation but performance doubled in Chrome 28: https://en.wikipedia.org/wiki/Asm.js#Implementations.
#2: Supported in MS Edge under the "Enable experimental JavaScript features" flag.

Resources: Homepage, Source for spec and tools, Bringing Asm.js to Chakra and Microsoft Edge, Microsoft Edge support announcement, MDN article about asm.js, Wikipedia article about asm.js.
other1253.228793.152237
html5Audio Tracks

Method of specifying and selecting between multiple audio tracks. Useful for providing audio descriptions, director's commentary, additional languages, alternative takes, etc.

Browser-specific notes:
#1: Supported in Firefox by enabling "media.track.enabled" in about:config.
#2: Supported in Chrome and Opera by enabling "enable-experimental-web-platform-features" in chrome:flags.

Resources: MDN Web Docs - HTMLMediaElement.audioTracks.
ls2.1745796.143335
html5Video Tracks

Method of specifying and selecting between multiple video tracks. Useful for providing sign language tracks, burnt-in captions or subtitles, alternative camera angles, etc.

Browser-specific notes:
#1: Supported in Firefox by enabling "media.track.enabled" in about:config.
#2: Supported in Chrome and Opera by enabling "enable-experimental-web-platform-features" in chrome:flags.

Resources: MDN Web Docs - HTMLMediaElement.
ls2.1745796.143335
cssCSS background-attachment

Method of defining how a background image is attached to a scrollable element. Values include scroll (default), fixed and local.

Most mobile devices have a delay in updating the background position after scrolling a page with fixed backgrounds.

Browser-specific notes:
#1: Partial support refers to supporting fixed but not local.
#2: Partial support refers to supporting local but not fixed.
#3: Only supports local when -webkit-overflow-scrolling: touch is _not_ used.
#4: Does not support fixed, and due to a bug only supports local if a border-radius is set on the element.
#5: Broken support of fixed and local when scrolling an outer scroll container. Related to a bug.

Resources: MDN Web Docs - background-attachment.
cr4.4134915.452535
cssCSS text-wrap: balance

Allows multiple lines of text to have their lines broken in such a way that each line is roughly the same width, often used to make headlines more readable and visually appealing.

Browser-specific notes:
#1: Only supports text-wrap: balance, not text-wrap-style: balance.

Resources: WebKit support bug, Firefox support bug, Blog post, Polyfill.
wd1253.2‒17.017.541141143.1‒17.017.52312134
jsPointer Lock API

API that provides access to raw mouse movement data. This is done by ignoring boundaries resulting from screen edges where the cursor can't go beyond, providing proper control for first person or real time strategy games.

Resources: MDN Web Docs - Pointer Lock, Simple demo.
rec2.13.2371310.145034
domDirectory selection from file input

The webkitdirectory attribute on the <input type="file"> element allows entire directory with file contents (and any subdirectories) to be selected.

Lack of support in mobile browsers may be due to the OS file picker not having support for selecting a directory.

Resources: MDN article.
unoff2.13.2301411.145033
cssCSS font-size-adjust

Method of adjusting the font size in a matter that relates to the height of lowercase vs. uppercase letters. This makes it easier to set the size of fallback fonts.

Browser-specific notes:
#1: Enabled through the "experimental Web Platform features" flag in chrome://flags.
#2: Missing support for two-value syntax. Issue for Chromium.
#3: Missing support for from-font value. Issue for Chromium.

Resources: Article on font-size-adjust, MDN Web Docs - CSS font-size-adjust, WebKit support bug #15257.
rec2.13.2‒16.01743120‒1251271173.117411833
otherCSS Paged Media (@page)

CSS at-rule (@page) to define page-specific rules when printing web pages, such as margin per page and page dimensions.

Currently no browsers appear to support the marks & bleed properties from the latest version of the specification.

Browser-specific notes:
#1: Does not support the size property.
#2: Does not appear to have a way to print web pages.

Resources: CSS Paged media article, MDN Web Docs - CSS @page, WebKit support bug, Firefox support bug.
wd2.13.215793.149532
jsFIDO U2F API

JavaScript API to interact with Universal Second Factor (U2F) devices. This allows users to log into sites more securely using two-factor authentication with a USB dongle.

The U2F API is being decommissioned (only the API, not the U2F protocol) and superseded by WebAuthn.

Browser-specific notes:
#1: Requires the "FIDO U2F (Universal 2nd Factor)" Chrome extension.
#2: Support can be enabled with the security.webauth.u2f flag.
#3: Supported via the internal CryptoTokenExtension..

Resources: Mozilla bug, Google Security article, Chrome platform status: U2F Security Key API removal (Cryptotoken Component Extension), Yubico blog post about the decommission, Chromium Intent to Deprecate and Remove: U2F API (Cryptotoken), Mozilla Firefox bug to remove the U2F API.
unoff2.113.310610613411432
cssCSS Container Style Queries

Style queries in Container Queries provide a way to query the current styling of a container, and conditionally apply additional CSS to the contents of that container.

Browser-specific notes:
#1: Enabled through the "Experimental Web Platform features" flag in chrome://flags.
#2: Partial support refers to only working with CSS custom property values in the style() query.

Resources: Getting Started with Style Queries, Style Queries, Container Queries: Style Queries, CSS Style Queries, Firefox support bug, WebKit support bug.
wd1253.2‒17.0181071111113.1‒17.01822231
otherDocument Policy

A mechanism that allows developers to set certain rules and policies for a given site. The rules can change default browser behaviour, block certain features or set limits on resource usage. Document Policy is useful both for security and performance, and is similar to Permissions Policy.

Standard support includes the HTTP Document-Policy header and policy attribute on iframes.

Browser-specific notes:
#1: Chromium browsers only support the HTTP header.

Resources: Firefox position: non-harmful, Chromium tracking bug for new policies, WICG - Document Policy Explainer.
unoff1253.285853.14230
otherSPDY protocol

Networking protocol for low-latency transport of content over the web. Superseded by HTTP version 2.

See also support for HTTP2, successor of SPDY.

Browser-specific notes:
#1: Deprecated as of macOS Mojave 10.4.4. To be completely removed in a future version of macOS.
#2: Deprecated as of iOS 12.2. To be completely removed in a future version of iOS.

Resources: Wikipedia, SPDY whitepaper.
unoff12512.2511212.155130
svgSVG fonts

Method of using fonts defined as SVG shapes. Removed from SVG 2.0 and considered as a deprecated feature with support being removed from browsers.

Browser-specific notes:
#1: Chrome 38 and newer support SVG fonts only on Windows Vista and XP.
#2: Supported in Opera Mini in SVG images only, not in HTML.

Resources: Blog post, Blog post on usage for iPad.
Parent feature: @font-face Web fonts.
rec1253.25193.25230
cssCSS filter() function

This function filters a CSS input image with a set of filter functions (like blur, grayscale or hue).

Resources: Blog post, Firefox support bug, Chromium support bug.
wd2.11045.59.14229
cssCSS hanging-punctuation

Allows some punctuation characters from start (or the end) of text elements to be placed "outside" of the box in order to preserve the reading flow.

Resources: CSS tricks article, Firefox bug #1253615, Chrome bug #41491716.
cr2.11045.5104229
otherJPEG 2000 image format

JPEG 2000 was built to supersede the original JPEG format by having better compression and more features. WebP, AVIF and JPEG XL are all designed to supersede JPEG 2000.

Browser-specific notes:
#1: Not supported by Safari for Windows.

Resources: Wikipedia article.
other2.15.0‒17.01845.56‒17.0184229
cssui-serif, ui-sans-serif, ui-monospace and ui-rounded values for font-familywd2.113.445.513.14229
html5Custom protocol handling

Method of allowing a webpage to handle a given protocol using navigator.registerProtocolHandler. This allows certain URLs to be opened by a given web application, for example mailto: addresses can be opened by a webmail client.

Browser-specific notes:
#1: Supports protocols mailto, mms, nntp, rtsp, and webcal but requires custom protocols to start with web+.

Resources: MDN Web Docs - Register protocol handler.
ls2.13.213793.14329
jsWebUSB

Allows communication with devices via USB (Universal Serial Bus).

Resources: Google Developers article, Mozilla Specification Positions: Harmful.
unoff2.13.261793.18.2227
cssCSS Canvas Drawings

Method of using HTML5 Canvas as a background image. Not currently part of any specification.

A similar effect can be achieved in Firefox 4+ using the -moz-element() background property.

Resources: WebKit blog post.
unoff1253.2485.545227
jsWebGPU

An API for complex rendering and compute, using hardware acceleration. Use cases include demanding 3D games and acceleration of scientific calculations. Meant to supersede WebGL.

All major browser engines are working on implementing this spec.

Browser-specific notes:
#1: Can be enabled in Firefox with the dom.webgpu.enabled flag.
#2: Can be enabled in Safari on MacOS with the WebGPU experimental feature.
#3: Can be enabled in some Chromium browsers (on Windows, MacOS, Linux) with the enable-unsafe-webgpu flag.
#4: Part of an origin trial.
#5: Not enabled on Linux by default.

Resources: Implementation status, Official Wiki, WebGPU test scene.
wd2.13.29411311315‒17.0TP246327
jsSpeech Recognition API

Method to provide speech input in a web browser.

Browser-specific notes:
#1: Partial support refers to some attributes missing.
#2: Firefox currently has a media.webspeech.recognition.enable flag in about:config for this, but actual support is waiting for permissions to be sorted out.
#3: Edge and Opera appear to have support for the SpeechRecognition API but no events for it appear to fire.
#4: Safari 14.1 and TP 119+ include prefixed support for webkitSpeechRecognition without support for SpeechGrammar or SpeechGrammarList. Siri needs to be enabled. Not available in SafariViewController and web apps added to Home Screen.

Resources: HTML5Rocks article, SitePoint article, Demo, Advanced demo and resource, Chromium bug to unprefix the Speech Recognition API.
unoff2.114.5257914.142226
jsFilesystem & FileWriter API

Method of reading and writing files to a sandboxed file system.

The File API: Directories and System specification is no longer being maintained and support may be dropped in future versions.

Resources: HTML5 Rocks tutorial, WebPlatform Docs, Firefox tracking bug.
unoff2.13.213793.15224
jsMagnetometer

Defines a concrete sensor interface to measure magnetic field in the X, Y and Z axis.

Browser-specific notes:
#1: Available by enabling the "Generic Sensor Extra Classes" experimental flag in about:flags.

Resources: Demo, Article.
cr2.13.267793.14224
otherJPEG XL image format

A modern image format optimized for web environments. JPEG XL generally has better compression than WebP, JPEG, PNG and GIF and is designed to supersede them. JPEG XL competes with AVIF which has similar compression quality but fewer features overall.

Browser-specific notes:
#1: Can be enabled via the enable-jxl flag.
#2: Can be enabled via the image.jxl.enabled flag in about:config in Nightly only. This flag is configurable but has no effect in the other builds.
#3: Can be enabled via the --enable-features=JXL runtime flag.
#4: Supports still images. Animated image sequences are not supported.

Resources: Official website, Comparison to other formats, Chromium support bug, Firefox support bug, WebKit support bug.
other2.13.2‒16.017911101103.11749024
jsWeb Serial API

Allows communication with devices via a serial interface.

Resources: Explainer, Read from and write to a serial port, Mozilla position: harmful, WebKit position: opposed.
unoff2.13.289893.14224
jsWebHID API

Enables raw access to HID (Human Interface Device) commands for all connected HIDs. Previously, an HID could only be accessed if the browser had implemented a custom API for the specific device.

Resources: Human interface devices on the web: a few quick examples.
unoff2.13.289893.14224
otherHEIF/HEIC image format

A modern image format based on the HEVC video format. HEIC generally has better compression than WebP, JPEG, PNG and GIF. It is hard for browsers to support HEIC because it is complex and expensive to license. AVIF and JPEG XL provide free licenses and are designed to supersede HEIC.

Browser-specific notes:
#1: Was supported natively in macOS/iOS, but was not supported in Safari.

Resources: Wikipedia article, Firefox support bug, WebKit support bug.
other2.111.0‒16.01745.511174223
otherXHTML+SMIL animation

Method of using SMIL animation in web pages.

Internet Explorer supports the W3C proposal HTML+TIME, which is largely the same as XHTML+SMIL.

Resources: Wikipedia, JS library to support XHTML+SMIL.
Parent feature: XHTML served as application/xhtml+xml.
unoff2.13.24793.14219
cssCSS text-justify

CSS property to define how text should be justified when text-align: justify is set.

Browser-specific notes:
#1: Supports inter-word, but not inter-character or none. Also supports the following unofficial values: distribute, distribute-all-lines, distribute-center-last, inter-cluster, inter-ideograph, newspaper. See MDN for details.
#2: inter-word and distribute values supported behind the "Experimental platform features" flag but distribute support is buggy.
#3: Behind the "layout.css.text-justify.enabled" flag.
#4: Supports auto, none, inter-word, inter-character, and distribute with the exact same meaning and behavior as inter-character for legacy reasons.

Resources: Chrome support bug, WebKit support bug, Firefox support bug.
cr1253.243793.155517
css3CSS font-smooth

Controls the application of anti-aliasing when fonts are rendered.

Though present in early (2002) drafts of CSS3 Fonts, font-smooth has been removed from this specification and is currently not on the standard track.

Browser-specific notes:
#1: WebKit implements something similar with a different name -webkit-font-smoothing and different values: none, antialiased and subpixel-antialiased.
#2: Firefox implements something similar with a different name -moz-osx-font-smoothing and different values: auto, inherit, unset, grayscale.
#3: Works only on Mac OS X platform.
#4: Firefox 128 and later supports -webkit-font-smoothing: antialiased (by mapping to -moz-osx-font-smoothing: grayscale).

Resources: MDN Web Docs - font-smooth, Old version of W3C recommendation containing font-smooth, WHATWG compat issue to spec `-webkit-font-smoothing: antialiased`.
unoff2.13.2579442512816
cssCSS -webkit-user-drag property

The non-standard -webkit-user-drag CSS property can be used to either make an element draggable or explicitly non-draggable (like links and images). See the standardized draggable attribute/property for the recommended alternative method of accomplishing the same functionality.

Webkit and blink-based mobile browsers recognize the property but it does not appear to have any effect.

There is currently no unprefixed user-drag or other version of the property implemented in browsers or on a standards track as the HTML draggable attribute/property is the preferred solution.

Browser-specific notes:
#1: -webkit-user-drag: none works, but -webkit-user-drag: element does not appear to have an effect.

Resources: Reference.
unoff2.13.216793.14216
jsWebXR Device API

API for accessing virtual reality (VR) and augmented reality (AR) devices, including sensors and head-mounted displays.

Browser-specific notes:
#1: This API is very extensive. Many of its features are still in development and/or don't have finalised specs yet.
#2: Can be enabled in Firefox behind the dom.vr.webxr.enabled flag.
#3: Can be enabled in Safari with the WebXR Device API experimental feature.

Resources: MDN Web Docs - WebXR Device API, Immersive Web - WebXR samples, Safari implementation bug.
cr2.13.2797913127715
otherOgg/Theora video format

Free lossy video compression format.

Browser-specific notes:
#1: Chrome is deprecating and removing Theora support, see this platform status entry.

Resources: Wikipedia article, Chrome Platform Status: Deprecate and remove Theora support, Firefox bug: Investigate removing Theora support.
Parent feature: Video element.
other2.13.241201223.143.512
jsPermissions Policy

A security mechanism that allows developers to explicitly enable or disable various powerful browser features for a given site. Similar to Document Policy.

Standard support includes the HTTP Permissions-Policy header, allow attribute on iframes and the document.permissionsPolicy JS API.

Browser-specific notes:
#1: Chromium browsers only support the HTTP header.
#2: At least partially supports Feature Policy, the predecessor to this spec.

Resources: W3C - Permissions Policy Explainer, Firefox implementation tracker, List of known features.
wd12511.3888811.18.27412
domHTML Imports

Deprecated method of including and reusing HTML documents in other HTML documents. Superseded by ES modules.

Resources: HTML5Rocks - HTML Imports: #include for the web, Chromium tracking bug: Implement HTML Imports, Firefox tracking bug: Implement HTML Imports, IE Web Platform Status and Roadmap: HTML Imports.
wd2.178080613567
jsWeb SQL Database

Method of storing data client-side, allows SQLite database queries for access and manipulation.

The Web SQL Database specification is no longer being maintained and support is intended to be dropped in future versions. Migrate to e.g. Web Storage or IndexedDB.

Browser-specific notes:
#1: Not available in third-party contexts.
#2: Requires a secure context.
#3: Requires a deprecation Origin Trial token.

Resources: HTML5 Doctor article, has.js test, Chrome platform status: Deprecate and remove WebSQL in non-secure contexts.
unoff1251310111912412413426
jsAmbient Light Sensor

Defines a concrete sensor interface to monitor the ambient light level or illuminance of the device’s environment.

Browser-specific notes:
#1: Implements an outdated version of the spec.
#2: Available by enabling the "Generic Sensor Extra Classes" experimental flag in about:flags.

Resources: Demo, Article, MDN Web Docs - Ambient Light Sensor.
cr2.13.258793.14606
domCustom Elements (deprecated V0 spec)

Original V0 version of the custom elements specification. See Custom Elements V1 for support for the latest version.

Resources: Blog post on upgrading from V0 to V1.
unoff12578080613596
jsWebVR API

API for accessing virtual reality (VR) devices, including sensors and head-mounted displays. Replaced by the WebXR Device API.

For a WebVR experience a head mounted display (VR HMD) is required.

Browser-specific notes:
#1: Available and enabled by default only in Firefox Windows. Enabled in Nightly for iOS. Firefox 73+ requires a secure context using HTTPS.
#2: Was in development in the latest Edge builds and supported only Windows Mixed Reality.
#3: Supports only Samsung Galaxy devices with the Samsung Gear VR.

Resources: Detailed device support information, WebVR polyfill, WebVR framework, WebVR info, MDN Web Docs - WebVR API, Chrome Platform Status for WebXR Device API.
unoff2.13.280803.15555
cssCSS Device Adaptation

Method of overriding the size of viewport in web page using the @viewport rule, replacing Apple's own popular <meta> viewport implementation. Includes the extend-to-zoom width value.

Due to lack of implementation this specification is slated to be retired.

Browser-specific notes:
#1: IE only supports the 'width' and 'height' properties.
#2: Opera Mobile and Opera Mini only support the 'orientation' property.

Resources: Introduction to meta viewport and @viewport in Opera Mobile, Device adaptation in Internet Explorer 10, Chrome tracking bug, WebKit tracking bug, Mozilla tracking bug.
wd2.13.229793.1425
jsProximity API

Defines events that provide information about the distance between a device and an object, as measured by a proximity sensor.

Resources: Demo, SitePoint article.
cr2.13.245.53.14154
html5Portals

Portals enable seamless navigation between sites or pages. A new page can be loaded as an inset using the <portal> element (similar to an iframe) which can then seamlessly transition to the new navigated state when "activated".

Browser-specific notes:
#1: Available when registered for origin trial.
#2: Available by enabling the #enable-portals flag at chrome://flags/.

Resources: Hands-on with Portals: seamless navigation on the Web.
unoff2.13.286903.1424
cssCSS @scroll-timeline

CSS property that allows animations to be driven by a container's scroll position.

Browser-specific notes:
#1: Can be enabled in Chrome via the #enable-experimental-web-platform-features flag in chrome://flags.
#2: Can be enabled in Chrome via the --additional-blink-features=CSSScrollTimeline build flag.

Resources: Demo, ScrollTimline examples, Firefox support bug, WebKit support bug, Scroll-timeline Polyfill, CSS-tricks article.
unoff2.13.288903.1424
html5Selectlist - Customizable select element

Proposal for a customizable <select> element, currently defined as <selectlist>, previously <selectmenu>.

Browser-specific notes:
#1: Available behind the "Experimental Web Platform features" flag. Implemented as selectmenu before Chromium 119.

Resources: Blog post: Two levels of customising <selectlist>, Open UI's <selectlist> demos.
unoff2.13.297973.1424
cssCSS text-box-trim & text-box-edge

Provides the ability to remove the vertical space appearing above and below text glyphs, allowing more precise positioning and alignment.

Previously specified as the leading-trim & text-edge properties.

Browser-specific notes:
#1: Available behind a Safari feature flag.

Resources: Document with examples of text-box-trim uses, CSS Tricks article.
wd2.13.2‒16.016.445.53.116.4TP424
css3CSS element() function

This function renders a live image generated from an arbitrary HTML element.

Browser-specific notes:
#1: In Firefox < 4, usage is limited to the background and background-image CSS properties.

Resources: MDN Web Docs - CSS element.
wd2.13.245.53.1444
cssCSS overflow: overlay

The overlay value of the overflow CSS property is a non-standard value to make scrollbars appear on top of content rather than take up space. This value is deprecated and related functionality being standardized as the scrollbar-gutter property.

Browser-specific notes:
#1: The overlay value is recognized, but behaves the same as "auto".

Resources: MDN article on overflow values, WebKit change to make "overflow: overlay" a synonym for "overflow: auto".
unoff125121511411412424
html5Offline web applications

Now deprecated method of defining web page files to be cached using a cache manifest file, allowing them to work offline on subsequent visits to the page.

This technology is being deprecated in favor of Service Workers.

Resources: Sitepoint tutorial, Dive Into HTML5 article, Mozilla Hacks article/demo, WebPlatform Docs.
unoff125158585154843
domShadow DOM (deprecated V0 spec)

Original V0 version of the Shadow DOM specification. See Shadow DOM V1 for support for the latest version.

Resources: HTML5Rocks - Shadow DOM 101 article, Safari implementation bug, Firefox implementation bug, Google Developers - Shadow DOM v1: self-contained web components.
unoff1253.280803.113610
securityHTTP Public Key Pinning

Declare that a website's HTTPS certificate should only be treated as valid if the public key is contained in a list specified over HTTP to prevent MITM attacks that use valid CA-issued certificates.

All browsers have removed support. The header was too complicated to use, and when incorrectly implemented, could completely block websites for longer periods of time.

Certificate transparency is widely used and tries to provide the same security by very different means.

Resources: MDN Web Docs - Public Key Pinning, Scott Helme article on the issues of HPKP.
other2.13.2725.53.111.1720
cssdisplay: run-in

If the run-in box contains a block box, same as block. If a block box follows the run-in box, the run-in box becomes the first inline box of the block box. If an inline box follows, the run-in box becomes a block box.

Browser-specific notes:
#1: Not before inline-elements.

Resources: Mozilla bug report, CSS Tricks article.
cr4.4.3832127420
otherJPEG XR image format

JPEG XR was built to supersede the original JPEG format by having better compression and more features. WebP, AVIF and JPEG XL are all designed to supersede JPEG XR.

Resources: Microsoft JPEG XR Codec Overview, Firefox support bug, Chrome support bug (marked as WONTFIX).
other2.13.24793.1420
jsEfficient Script Yielding: setImmediate()unoff2.13.24793.1420
domResource Hints: Lazyload

Gives a hint to the browser to lower the loading priority of a resource. Please note that this is a legacy attribute, see the loading attribute for the new standardized API.

Resources: lazyload attribute | lazyload property, Discussion on standardization.
unoff2.13.24793.1420
otherEOT - Embedded OpenType fonts

Type of font that can be derived from a regular font, allowing small files and legal use of high-quality fonts. Usage is restricted by the file being tied to the website.

Proposal by Microsoft, being considered for W3C standardization.

Resources: Wikipedia, Example pages.
Parent feature: @font-face Web fonts.
unoff2.13.24123.1420
cssCSS Exclusions Level 1

Exclusions defines how inline content flows around elements. It extends the content wrapping ability of floats to any block-level element.

Resources: CSS Exclusions, Firefox tracking bug, WebKit tracking bug, Chromium tracking bug.
wd2.13.24793.1420
jsInput Method Editor API

Provides scripted access to the Input Method Editor (IME). An IME is often used to input characters from East Asian languages by typing roman characters and selecting from the resulting suggestions.

Browser-specific notes:
#1: Prefixed implementation uses msGetInputContext() method from an earlier version of the spec to get the InputMethodContext, rather than the inputMethodContext attribute. Also includes support for getCompositionAlternatives() from the latest Editor'.

Resources: Building Better Input Experience for East Asian Users with the IME API in IE11.
unoff2.13.24793.1420
css3CSS Regions

Method of flowing content into multiple elements, allowing magazine-like layouts. While once supported in WebKit-based browsers and Internet Explorer, implementing the feature is no longer being pursued by any browser.

Browser-specific notes:
#1: Support is limited to using an iframe as a content source with the -ms-flow-into: flow_name; and -ms-flow-from: flow_name; syntax.
#2: Partial support refers to not supporting the region-fragment property.

Resources: Adobe demos and samples, IE10 developer guide info, Firefox feature request bug, Beginner's guide to CSS Regions, Discussion on removal in Blink.
wd2.111.3357911.1420
jsWeb NFC

This API allows a website to communicate with NFC tags through a device's NFC reader.

Many devices are not equipped with NFC readers. They won't return any data, even though an installed browser might support this API.

Browser-specific notes:
#1: Was experimentally supported in desktop Chromium browsers behind the enable-experimental-web-platform-features flag.
#2: Current implementations only allow communication through the NDEF protocol in order to minimize security and privacy issues.

Resources: Safari position: Opposed, Firefox position: Harmful, Web.dev article on using WebNFC.
unoff2.13.289893.1420
cssScoped attribute

Deprecated method of allowing scoped CSS styles using a "scoped" attribute. Now removed from the specification and replaced by the @scope CSS rule.

Browser-specific notes:
#1: Enabled in Chrome through the "experimental Web Platform features" flag in chrome://flags.
#2: Enabled in Firefox through the about:config setting "layout.css.scoped-style.enabled".

Resources: Polyfill, HTML5 Doctor article, HTML5Rocks article, Firefox bug #1291515: disable `<style scoped>` in content documents.
unoff2.13.2375.53.14610
jsObject.observe data binding

Method for data binding, a now-withdrawn ECMAScript 7 proposal.

Support in Blink-based browsers is expected to be removed in future versions.

Resources: Data-binding Revolutions with Object.observe(), Polyfill, Firefox tracking bug, An update on Object.observe.
unoff2.13.2505.53.1520
otherDynamic Adaptive Streaming over HTTP (MPEG-DASH)

HTTP-based media streaming communications protocol, an alternative to HTTP Live Streaming (HLS).

DASH can be used with a JavaScript library in browsers that doesn't support it natively as long as they support Media Source Extensions.

Browser-specific notes:
#1: Can be enabled via the media.dash.enabled flag.
#2: Only WebM video is supported.

Resources: Wikipedia article, JavaScript implementation.
other2.13.24793.14230
jsObject RTC (ORTC) API for WebRTC

Enables mobile endpoints to talk to servers and web browsers with Real-Time Communications (RTC) capabilities via native and simple JavaScript APIs.

ORTC is often dubbed WebRTC 1.1. It is possible to make ORTC communicate with WebRTC 1.0 endpoints. See WebRTC 1.0 for support details for that API.

Resources: Related blog posts by Microsoft.
other2.13.24793.1420
html5seamless attribute for iframes

The seamless attribute makes an iframe's contents actually part of a page, and adopts the styles from its hosting page. The attribute has been removed from both the WHATWG and the W3C HTML5 specifications.

Chrome 20-26 had partial support behind a flag, though this was later removed.

Browser-specific notes:
#1: Hides the border of seamless iframes and recognizes the 'seamless' DOM property, but does not provide actual support.

Resources: Experimental polyfill, Article, Bug on Firefox support: wontfix.
unoff2.18275.58420
cssExplicit descendant combinator >>unoff2.13.248011.1420
jsTemporal

A modern API for working with date and time, meant to supersede the original Date API.

All browsers are working on implementing this.

Resources: Fixing JavaScript Date, Chromium implementation bug, Firefox implementation bug, WebKit implementation bug, Blog post: Temporal: getting started with JavaScript’s new date time API.
other2.13.245.53.1420
jsDecorators

ECMAScript Decorators are an in-progress proposal for extending JavaScript classes. Decorators use a special syntax, prefixed with an @ symbol and placed immediately before the code being extended.

While not yet supported natively in browsers, decorators are supported by a number of transpiler tools.

Resources: JavaScript Decorators: What They Are and When to Use Them, A minimal guide to JavaScript (ECMAScript) Decorators and Property Descriptor of the Object, Decorators in TypeScript, Babel plug-in for decorators, Bug on Firefox support.
unoff2.13.245.53.1420
cssCSS3 attr() function for all properties

While attr() is supported for effectively all browsers for the content property, CSS Values and Units Level 5 adds the ability to use attr() on **any** CSS property, and to use it for non-string values (e.g. numbers, colors).

See the generated content table for support for attr() for the content property.

Resources: MDN Web Docs - CSS attr, Mozilla Bug #435426: implement css3-values extensions to `attr()`, Chromium issue #246571: Implement CSS3 attribute / attr references, WebKit Bug #26609: Support CSS3 attr() function.
unoff2.13.245.53.1420
cssCSS @when / @else conditional rules

Syntax allowing CSS conditions (like media and support queries) to be written more simply, as well as making it possible to write mutually exclusive rules using @else statements.

Resources: Blog post: Extending CSS when/else chains: A first look, Chrome support bug, Firefox support bug, WebKit support bug.
wd2.13.245.53.1420

supported annotated partial optional missing unknown prefixed

Usage percentage: 0 - .01 - 1-9 - 10 - 20 - majority