1. 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.

  2. CSS 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.

  3. CSS 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.

  4. CSS initial value

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

  5. CSS Motion Path

    Allows elements to be animated along SVG paths or shapes via the `offset-path` property. Originally defined as the `motion-path` property.

  6. CSS 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.

  7. CSS 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`.

  8. CSS Scroll Snap

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

  9. CSS 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)

  10. Custom Elements (V1)

    One of the key features of the Web Components system, custom elements allow new HTML tags to be defined.

  11. CustomEvent

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

  12. ES6 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.

  13. CSS font-variant-alternates

    Controls the usage of alternate glyphs associated to alternative names defined in @font-feature-values for certain types of OpenType fonts.

  14. accept 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

  15. Proxy object

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

  16. Selectlist - Customizable select element

    Proposal for a customizable `<select>` element, currently defined as `<selectlist>`, previously `<selectmenu>`.

  17. SVG fonts

    Method of using fonts defined as SVG shapes. Removed from [SVG 2.0](https://www.w3.org/TR/SVG2/changes.html#fonts) and considered as a deprecated feature with support being removed from browsers.

  18. text-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`)

  19. Variable 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.

  20. Small, 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 (`svb`, `svh`, `svi`, `svmax`, `svmin`, `svw`), `lv*` units (`lvb`, `lvh`, `lvi`, `lvmax`, `lvmin`, `lvw`), `dv*` units (`dvb`, `dvh`, `dvi`, `dvmax`, `dvmin`, `dvw`) and the logical `vi`/`vb` units.

  21. customelementregistry api: whendefined

  22. css selector: `:defined`

  23. javascript built-in: undefined