Intent to Prototype: "Customizable Select"

[Keith Cirkel]

Summary:

  Prototype out the various extensions to select elements, including:

  - The `<selectedcontent>` element.

  - Relaxing of the parser restrictions & content model for `<select>` elements.

  - Defining `appearance: base-select`, and implementing the UA Stylesheet and ShadowDOM changes relating to this new mode.

  - Defining new pseudo elements such as `::picker(select)`, `::picker-icon` & `::checkmark`.

  - New ARIA mappings.

Bug:

  https://bugzilla.mozilla.org/show_bug.cgi?id=1944403

Specification:

  Various specifications include this work:

  - CSS Forms 1 (https://drafts.csswg.org/css-forms-1/)

  - CSS UI 4 (https://drafts.csswg.org/css-ui-4/)

  - CSS Pseudo 4 (https://drafts.csswg.org/css-pseudo-4/)

  - HTML (https://html.spec.whatwg.org/)

  - HTML-AAM (https://w3c.github.io/aria/html-aam/)

Standards Body:

  W3C (WHATWG, CSSWG, ARIAWG)

Platform coverage:

  all.

Preference:

  A variety of feature flags will be implemented so that each facet can be independently verified and/or shipped, including:

  - dom.lift-select-parser-restrictions.enabled (for the new parser/content model changes)

  - dom.selectedcontent.enabled (for the new `<selectecontent>` element)

  - layout.css.select-pseudo-elements.enabled (for the new pseudo elements)

DevTools bug:

  N/A

Link to standards-positions discussion:

  https://github.com/mozilla/standards-positions/issues/1060 general position; positive.

  https://github.com/mozilla/standards-positions/issues/1142 for `<selectedcontent>`.

  https://github.com/mozilla/standards-positions/issues/1086 for relaxing of the parser; positive.

  

Other browsers:

  Blink: Positive, shipping.

  WebKit: Supportive.

web-platform-tests:

  Many in https://wpt.fyi/results/html/semantics/forms/the-select-element/customizable-select.

Myself and other members of the DOM Core team at Mozilla will be prototyping out these features, which collectively have been called "Customizable Select". There is a large body of work here, and so we may work on (or enable in nightly) individual parts (such as lifting the parser restrictions) while ensuring web compatibility is maintained.