The :open CSS pseudo-class matches elements that have open states, like <details>, <dialog>, or <select>, based on their state.

The source property of a ToggleEvent object is the element which triggered the toggle event to fire for a popover, <dialog>, or <details> element, if applicable.

The image-rendering: crisp-edges CSS declaration scales images to preserve lines without blurring.

The text-decoration-skip-ink: all CSS declaration forces interruptions in underlines and overlines where the line would cross a glyph. This contrasts with auto, which does not skip for CJK glyphs.

The SharedWorker() constructor accepts { type: "module" } to load scripts that use import and export. Also known as ECMAScript modules or ESM in shared workers.

The SharedWorker() constructor runs a script in its own thread, which can send and receive messages with other scripts running at the same origin.

The contrast-color() CSS function picks a color that has guaranteed contrast against a specified foreground or background color.

The Math.sumPrecise() static method returns the sum of an iterable of numbers. It avoids the precision loss of intermediate partial sums, as found using reduce() or a loop to add together an array of values.

The baseline-shift CSS property sets the position of an element relative to its dominant baseline.

The font-family: math CSS declaration uses the browser default font face for displaying mathematical expressions.

Custom highlights style arbitrary text ranges, without adding extra elements to the DOM.

The Iterator.concat() JavaScript method returns an iterator that yields values from a sequence of iterators, exhausting each iterator before moving on to the next.

A ReadableStream constructed with { type: "bytes" } reads bytes from a stream without making extra copies, improving efficiency for streams of large chunks. Also known as BYOB or bring your own buffer.

The Reporting-Endpoints HTTP header and ReportingObserver() API send selected reports, such as Content Security Policy (CSP) violation reports or crash reports, to a nominated URL or callback function.

CSP violation reporting sends a report to a URL nominated by the Reporting-Endpoints header or the ReportingObserver API when a page violates its content security policy.

The WebTransport API transmits data between a client and a server, by using the HTTP/3 protocol.

The text-indent: each-line CSS declaration indents text after forced breaks as well as to the first line of a block.

The text-indent: hanging CSS declaration indents all lines except the first.

The shape() CSS function creates shapes with a series of commands like line, move, and curve. It can be used with clip-path and shape-outside.

Trusted types allow you to lock down insecure parts of the DOM API and prevent client-side cross-site scripting (XSS) attacks.

Branch hints in WebAssembly allows a browser to optimize performance when a branch is a likely to take a specific path.

The getOrInsert() and getOrInsertComputed() methods of Map objects get a value, setting and getting a default value if needed.

Zstandard or zstd is a fast lossless compression algorithm. When used as a content encoding, it is often faster and offers better compression than brotli.

The :active-view-transition CSS pseudo-class matches the root element when a view transition is active. The :active-view-transition-type() CSS pseudo-class matches only when the active view transition was started with the specified type.

The navigator.serviceWorker.register() method accepts { type: "module" } to load scripts that use import and export. Also known as ECMAScript modules or ESM in service workers.

The navigation API initiates, intercepts, or modifies browser navigation actions. Not to be confused with the navigator API.

The rcap CSS length unit is a font-relative length equal to the value of the cap unit on the root element. Cap-height is approximately equal to the height of a capital Latin letter.

The rch CSS length unit is a font-relative length equal to the value of the ch unit on the root element. ch length is based on the width of the zero (0) character.

The rex CSS length unit is a font-relative length that is equal to the x-height of the root element.

The ric CSS length unit, or root international character, is a font-relative length equal to the width of CJK character relative to the root element.

The document.caretPositionFromPoint() method finds an insertion point, represented by a DOM node and an offset within that node, for given coordinates in the viewport.

The event and first-input performance entries and the PerformanceEventTiming API measures the latency of user input events, such as mouse clicks or keypresses. They're used to calculate Interaction to Next Paint (INP), a common metric for perceived responsiveness.

The command and commandfor attributes for the <button> HTML element dispatch an action to an element when the button is invoked (by click or keystroke), as a declarative alternative to addEventListener() calls or onclick attribute handlers.

The largest-contentful-paint performance entry and the LargestContentfulPaint API measures the time it takes for the largest image or text to appear. Largest contentful paint (LCP) is a common metric for perceived loading times.

The @scope CSS at-rule sets the scope for a group of rules.

The scrollbar-color CSS property sets the color of the scrollbar track and thumb.

The scrollend event fires when an element or document has finished scrolling.

The text-decoration-line: spelling-error and text-decoration-line: grammar-error CSS declarations apply the browser's marking for spelling and grammatical mistakes. This is typically a wavy underline in red or green.

The Atomics.waitAsync() static method waits for a value in a shared memory location, providing a promise when the expected value is not yet in memory. The waitAsync() method is a non-blocking alternative to Atomics.wait().

The view-transition-class CSS property sets a name that can be used to apply styles to multiple named view transition pseudo-elements.

View transitions allow you to create animated visual transitions between different states of a document.

The WebRTC encoded transform API allows you to modify audio and video streams in WebRTC connections. For example, it can be used for visual effects or custom codecs.

The ::details-content pseudo-element selects the expandable content of a <details> element, excluding the <summary>.

The content-visibility CSS property delays rendering an element, including layout and painting, until it is needed.

The rel="dns-prefetch" attribute for the <link> HTML element is a hint to the browser that the page or user is likely to request resources from another domain, so the browser should preemptively resolve DNS for the href value's domain.

The Document.parseHTMLUnsafe() static method parses HTML into a DOM tree, while the setHTMLUnsafe() method of Element and ShadowRoot parses and inserts HTML into an existing tree. No sanitization applies to these methods, so never call them with user-provided HTML strings.

The URLPattern API creates patterns that can be matched against URLs or URL components.

The Uint8Array object methods fromBase64(), toBase64(), and setFromBase64() convert to and from base64 strings. The fromHex(), toHex(), and setFromHex() methods convert to and from hex strings.

The window.getSelection().getComposedRanges() method returns ranges that represent the current user selection, even if the selection spans across shadow tree boundaries.

The <input type="file" webkitdirectory> HTML element shows a file picker from which users can choose a folder to upload with the form.

The abs() and sign() CSS functions compute the absolute value or the sign of the input.

The requestClose() method of a <dialog> HTML element closes the dialog, firing a cancel event first, which listeners can use to prevent the dialog from closing. This differs from the close() method, which only fires the non-cancelable close event.

The print-color-adjust CSS property sets whether styles of printed pages should be adjusted to use less ink, in cases such as light text on a dark background.

The RegExp.escape() static method takes a string and replaces any characters that are potentially special characters of a regular expression with equivalent escape sequences. For example, RegExp.escape("[abc]") returns "\\[abc\\]".

Module import … with { type: "json" } statements load JSON data. Also known as JSON module scripts.

Float16Array is a typed array of 16-bit floating point numbers.

The Atomics.pause() static method gives a hint to the CPU that the code calling the method is in a short-duration wait for shared memory, known as spinning or a spinlock.

The ClipboardItem.supports() static method checks if the browser supports writing data types such as "image/svg+xml" or other custom formats to the system clipboard.

The Iterator object is an abstract base for objects that implement the iterator protocol. It provides methods common to built-in iterators, such as filter(), find(), map(), and reduce(). You can also use the static method Iterator.from() to convert an existing iterable into an Iterator.

To serialize and parse JSON in a lossless way, JSON.stringify() handles rawJSON values and JSON.parse()'s reviver callback takes a source context parameter.

The navigator.wakeLock.request("screen") API prevents the device's screen from dimming or being turned off.

The contenteditable="plaintext-only" global HTML attribute allows the user to edit the content of an element, but prevents rich-text formatting.

The Intl.DurationFormat API creates a locale-aware formatter that turns an object representing a duration (such as days, hours, and minutes) into a string.

The popover HTML attribute creates an overlay to display content on top of other page content. Popovers can be shown declaratively using HTML, or using the showPopover() method.

The Promise.try() static method returns a promise that takes a callback of any kind (returns or throws, synchronously or asynchronously) and wraps its result in a Promise.

The @page CSS at-rule sets the page-specific dimensions and margins for content such as printed documents, ebooks, or slides.

The ruby-align CSS property sets the spacing and alignment of ruby annotation text when it does not fill its available space.

The ruby-position CSS property sets the position of a ruby annotation in relation to its base text. Annotations can display over, under, or interleaved with the base text.

The scrollbar-gutter CSS property reserves space for the scrollbar, preventing unwanted layout changes as the scrollbar appears and disappears.

The scrollbar-width CSS property sets the width of the scrollbar.

The ::target-text pseudo-element allows you to style text highlighted by a URL text fragment such as #:~:text=snippet.

Garbage collection tracks references for better memory management with garbage-collected languages running in WebAssmbly.

Tail call optimization discards a caller frame and replaces the call with a jump instruction.

The fetch() priority option and the fetchpriority HTML attribute give hints to the browser about which requests to do before other requests of the same type.

The requestVideoFrameCallback() method for <video> schedules a function that runs with the next video frame. It is similar to requestAnimationFrame(), but for video.

The drawingBufferColorSpace and unpackColorSpace properties of WebGLRenderingContext sets the color representation for the buffer and imported textures, such as "srgb" or "display-p3".

The drawingBufferColorSpace and unpackColorSpace properties of WebGL2RenderingContext sets the color representation for the buffer and imported textures, such as "srgb" or "display-p3".

The text-wrap CSS property sets how lines break in text that overflows the container. It is a shorthand for text-wrap-style and text-wrap-mode.

Text fragments are URL fragments on the form #:~:text=snippet and link to a snippet of text within a page. The browser may scroll, highlight, or otherwise bring that text to the reader's attention.

The backdrop-filter CSS property applies graphical effects such as blurring or color shifting to the area behind an element.

The optional willReadFrequently parameter of a canvas's getContext() method permits the browser to optimize for frequent getImageData() calls by avoiding hardware acceleration. Also known as multiple readback.

The navigator.cookieEnabled property is a boolean for whether the browser accepts or ignores attempts to write cookie data.

The getHTML() method of Element and ShadowRoot objects serializes an element's DOM to an HTML string.

HTTP/3 is a major revision of the HTTP network protocol, providing improved performance and efficiency by using QUIC as the underlying transport protocol.

The from keyword for color functions (color(), hsl(), oklch(), etc.) creates a new color based on a given color by modifying the values of the input color. Also known as relative color syntax (RCS).

A typed function reference can be called directly.

Multiple <details> elements which use the same name attribute are mutually exclusive. When one member of the group is opened, all other members are closed.

The @starting-style CSS at-rule sets the starting values for properties that are transitioning when the target element's style is first updated.

The transition-behavior: allow-discrete CSS declaration allows transitions for properties whose animation behavior is discrete. Such properties can't be interpolated and swap from their start value to the end value at 50%.

The font-size-adjust CSS property preserves apparent text size, regardless of the font used, by scaling fonts to the same size with respect to a specific metric, such as x-height. This can help make fallback fonts look the same size.

The / notation in content CSS property values adds alternative text to generated content. For example, the content: url('upvote-icon.svg') / 'Upvote' declaration adds accessible 'Upvote' text to the SVG icon. Also known as alternate text.

The CSS.registerProperty() static method and the @property CSS at-rule register custom properties for which types and behaviors can be defined.

The resize() method of an ArrayBuffer and the grow() method of a SharedArrayBuffer, constructed with the maxByteLength option, changes the size of the buffer in place.

The navigator.clipboard API asynchronously reads and writes to the system clipboard.

Gradient interpolation controls how the colors between gradient stops are calculated in CSS. For example, linear-gradient(to right in oklch longer hue, red, red); calculates in the oklch color space, with the hue going all the way around the hue circle from red back to red.

The difference(), intersection(), isDisjointFrom(), isSubsetOf(), isSupersetOf(), symmetricDifference(), and union() methods of the JavaScript Set object performs operations between two sets.

The round(), mod(), and rem() CSS functions compute rounded values and the remainder after division.

The :state() CSS pseudo-class matches custom elements based on their custom state, set through the ElementInternals.states API.

The zoom CSS property scales the size of an element. Unlike the transform property, a zoomed element affects page layout.

The light-dark() CSS function accepts two colors and uses one depending on the current color scheme.

The text-wrap: balance CSS declaration balances the length of each line when text is broken into multiple lines. Also known as headline balancing.

The AbortSignal.timeout() static method returns an abort signal that automatically aborts after a specified duration.

The writing-mode CSS property orients form elements (such as radio buttons, progress bars, or select menus) vertically when the writing mode is vertical-lr or vertical-rl. The direction CSS property sets whether inputs flow from top to bottom or bottom to top.

The align-content property vertically aligns content in block layouts, like it does in flex or grid layouts.

The Intl.Segmenter API creates a locale-aware text splitter that can separate a string into meaningful graphemes, words, or sentences.

The transform-box CSS property sets the position and dimensions of the reference box relative to which an element's transformations are calculated.

The paint-order CSS property sets the z-order of strokes, fills, and (in SVG content) markers. For example, paint-order: markers stroke fill; draws the markers, then stroke on top of markers, then fill on top of both markers and stroke.

The AbortSignal.any() static method combines an iterable of abort signals into a single signal, with the abort reason taken from the first signal to abort.

The white-space-collapse CSS property sets whether new line characters are shown as line breaks, and whether multiple consecutive spaces are all displayed or combined.

The Object.groupBy() and Map.groupBy() static methods group values of arrays and iterables based on a function that returns a key for each value.

The checkVisibility() method checks if an element is potentially visible, with optional parameters for the kinds of visibility to check. For example, it checks whether the element has the style display: none, but can also check for visibility: hidden.

The Promise.withResolvers() static method is an alternative to the Promise() constructor that returns both the promise and resolution functions. You can use this to access resolve and reject outside the scope of the executor function.

The transfer() and transferToFixedLength() methods of ArrayBuffer move a buffer from one context to another (for example, to a worker).

The i32.add, i32.sub, i32.mul, i64.add, i64.sub, and i64.mul operations extend constant expressions to include arithmetic.

The shadowrootmode attribute on <template> creates a shadow root without the use of JavaScript. It is a declarative alternative to the attachShadow() method.

The Array.fromAsync() static method copies items from an async iterable object to make a new array.

AVIF (AV1 Image File Format) is an image format based on the AV1 video format.

The rect() CSS function creates a rectangle shape with insets from the edges of an element. The xywh() CSS function creates a rectangle shifted by an x and y distance. They can be used with clip-path and shape-outside.

The :has() CSS functional pseudo-class matches an element if any of the selectors passed as parameters would match at least one element.

The loading="lazy" attribute for <img> and <iframe> elements blocks loading the external resource until the user scrolls to that element's part of the page.

The reset() method clears a canvas to its initial state.

The cap CSS length unit corresponds to the height of Latin capital letters.

The counter-set CSS property creates (and optionally sets a value for) a counter, the numbers for a series of headings or ordered list items.

The createImageBitmap() global method creates an ImageBitmap object from a source such as an image, SVG, blob, or canvas. An ImageBitmap object represents pixel data that can be drawn to a canvas with lower latency than other types, such as ImageData.

The linear() easing function for animations and transitions interpolates linearly between the control points, and can be used to approximate complex easing functions, such as a bounce effect.

CSS nesting allows for shorter selectors, easier reading, and more modularity by nesting rules inside others.

The imagesrcset and imagesizes attributes with the rel="preload" attribute for the <link> HTML element starts fetching responsive images before they're found in the body of the document.

The preservesPitch property for <audio> or <video> adjusts the pitch of audio to sound more natural when the playback rate is faster or slower than the default.

The :dir() CSS functional pseudo-class matches elements by text direction, either right to left (rtl) or left to right (ltr).

The pow(), sqrt(), hypot(), log(), and exp() CSS functions compute various exponential functions.

The mask CSS property (and several longhand properties) partially or completely hides an element according to the shape and depth of an image.

The scripting CSS media query sets styles based on whether scripting such as JavaScript is available. Values are enabled if scripting is available, initial-only if scripting is only available on page load (for example, printed content), or none.

The URL.canParse() static method checks whether a URL can be parsed into a valid URL object. It's an alternative to calling new URL() in a try … catch statement.

The document.requestStorageAccess() method allows content in iframes to request storing and reading cookies and other site data, while the document.hasStorageAccess() method checks if such access is granted.

The lh CSS length unit corresponds to the requested line height, the computed value of the line-height property. Some lines may be higher than this based on their content.

The rlh CSS length unit is a font-relative length relative to the line height of root element.

The navigator.userActivation API reveals whether the user has interacted with the page through an "activation" gesture such as a click, tap, or key press. User activation gated APIs (such as the fullscreen API) fail without user interaction, and this API allows you to predict such a failure.

The getAuthenticatorData(), getPublicKey(), and getPublicKeyAlgorithm() methods of AuthenticatorAttestationResponse access credential data inside attestationObject without the need to parse it.