The initial keyword resets any CSS property to its initial value as defined by the specification. For example, the initial value of the display property is inline, regardless of the element being styled. Not to be confused with revert, which resets to the user or browser default style.

The <template> HTML element holds HTML fragments which you can clone and insert into the document using JavaScript.

Unicode point escapes (\\u{}) represent Unicode characters within strings.

Flexbox is a one-dimensional layout system, which places content either horizontally or vertically, with optional wrapping.

The CSS object model API reads, creates, and modifies CSS stylesheets and inline styles. Also known as CSSOM.

The word-break CSS property sets how lines break within words.

The transition shorthand CSS property sets how changes to an element's styles may occur over time. Transitions can be applied to specific CSS properties, all properties, or none.

A Symbol value is a unique, non-enumerable primitive value used for encapsulation or information hiding. For example, a symbol can be a key of an object that can never collide with any other key.

The animation CSS property animates an element's style over time, using keyframes described in @keyframes rules.

The codePointAt() method returns the numeric value of the UTF-16 code point at an index of the string. The fromCodePoint() method returns a string created from one or more code points.

The includes() method of strings returns whether a search string appears within the string.

The String.raw() static method interpolates template literal substitutions, but ignores escape sequences. It is the tag function for template literals.

The repeat() method of strings returns the original string repeated a number of times.

The startsWith() and endsWith() methods of strings returns whether a search string appears at the beginning or end of the provided string.

A WeakSet object stores unique values of any type without creating strong references to the values, such that membership in the set does not prevent garbage collection of the value.

The @supports at-rule applies styles based on a browser's support for CSS features, such as a CSS property and value. Also known as feature queries.

The HTTP/2 protocol is a major revision of the HTTP network protocol, providing improved performance and efficiency by using a single TCP connection to send multiple streams of data at once.

The transform CSS property and its 2D transform functions allow rotating, scaling, skewing, and translating an element. Arbitrary 2D transforms are also possible using a transformation matrix.

Channel messaging communicates one-to-one between browsing contexts on the same origin, such as between <iframe> elements or two documents using a shared worker.

The performance global object and the PerformanceObserver API provide access to performance-related information for the current execution context.

The dir global HTML attribute sets an element's text direction to left to right (ltr), right to left (rtl), or based on the content (auto). Elements generally inherit their parent's direction when the attribute is omitted.

The X-Frame-Options HTTP header limits where a document may be embedded in an iframe. The X-Frame-Options: SAMEORIGIN header disallows embedding in another origin. The X-Frame-Options: DENY header disallows embedding in any context, regardless of origin.

The display: table CSS declaration renders an element with the box layout of a <table> HTML element. Child elements may use equivalents to <table> internal elements such as display: table-row for <tr>.

The <meta http-equiv="refresh"> HTML element and the equivalent Refresh HTTP header trigger a page reload or redirect. They apply only when the page is fully loaded, after other HTTP redirects or scripted redirects that might execute before the page is fully loaded.

The BMP image format is an uncompressed raster graphics file format. Also known as a bitmap.

The GIF (Graphics Interchange Format) image format is a raster graphics file format with animation and lossless compression.

The JPEG image format is a raster graphics file format with lossy compression.

The <input type="file" multiple> HTML element shows a file picker from which users can choose one or more files to upload with a form.

The Worker() constructor runs a script in its own thread, which can send and receive messages with the script that created it. Also known as web workers.

The visible property of the window.locationbar, window.menubar, window.personalbar, window.scrollbars, window.statusbar, and window.toolbar APIs historically represented the visibility of a browser user interface. For privacy reasons, it now represents whether the window is a pop-up.