ch (character) unit

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

COLR/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.

CSS 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.

CSS Canvas Drawings

Method of using HTML5 Canvas as a background image. Not currently part of any specification.

Scoped Styles: the @scope rule

Allows CSS rules to be scoped to part of the document, with upper and lower limits described by selectors.

CSS 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.

CSS Masks

Method of displaying part of an element, using a selected image as a mask

seamless 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](https://github.com/whatwg/html/issues/331) and [the W3C](https://github.com/w3c/html/pull/325) HTML5 specifications.

Email, telephone & URL input types

Text input fields intended for email addresses, telephone numbers or URLs. Particularly useful in combination with [form validation](https://caniuse.com/#feat=form-validation)

Media 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.

Built-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.

Picture 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

'SameSite' cookie attribute

Same-site cookies ("First-Party-Only" or "First-Party") allow servers to mitigate the risk of CSRF and information leakage attacks by asserting that a particular cookie should only be sent with requests initiated from the same registrable domain.

SVG 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.

Signed 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.

URL API

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

cookiestore api: delete: `partitioned` option

cookiestore api: getall: `partitioned` in return value

cookiestore api: get: `partitioned` in return value

cookiestore api: set: `partitioned` option

element api: part

selection api: containsnode: `partialcontainment` parameter is optional

css selector: `::part`

html attribute: exportparts

html attribute: part

headers http header: set-cookie: `partitioned`

javascript built-in: intl: datetimeformat: formatrangetoparts

javascript built-in: intl: datetimeformat: formattoparts

javascript built-in: intl: durationformat: formattoparts

javascript built-in: intl: listformat: formattoparts

javascript built-in: intl: numberformat: formatrangetoparts

javascript built-in: intl: numberformat: formattoparts

javascript built-in: intl: relativetimeformat: formattoparts

lookahead assertions in js regular expressions

empty tag syntax: <>

css @apply rule