1. ch (character) unit

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

  2. 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.

  3. CSS Canvas Drawings

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

  4. 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.

  5. CSS Masks

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

  6. 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.

  7. 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)

  8. 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.

  9. Built-in PDF viewer

    Support for an PDF viewer that is part of the browser, rather than requiring a PDF file to be opened in an external application.

  10. 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

  11. '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.

  12. Scoped CSS

    Allows CSS rules to be scoped to part of the document, based on the position of the style element. The attribute has been [removed from the current specification](https://github.com/whatwg/html/issues/552).

  13. 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.

  14. 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 use with AMP CDNs, for example, to allow the original URL to be displayed in the URL bar.

  15. URL API

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

  16. element api: part

  17. selection api: containsnode: `partialcontainment` parameter is optional

  18. css selector: `::part`

  19. html attribute: exportparts

  20. html attribute: part

  21. javascript built-in: intl: datetimeformat: formatrangetoparts

  22. javascript built-in: intl: datetimeformat: formattoparts

  23. javascript built-in: intl: listformat: formattoparts

  24. javascript built-in: intl: numberformat: formattoparts

  25. javascript built-in: intl: relativetimeformat: formattoparts

  26. ecmascript 2021 (es12)

  27. lookahead assertions in js regular expressions

  28. empty tag syntax: <>