Intent to Ship: CustomStateSet

[Keith Cirkel]

Summary: Add support for `CustomStateSet` property

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1588763

Specification: https://html.spec.whatwg.org/multipage/custom-elements.html#custom-state-pseudo-class

Standards Body: CSS WG & HTML WG

Platform coverage: All

Preference: dom.element.customstateset.enabled

Devtools bug: N/A

Other browsers:

  - Blink: Shipped since Chromium 90, but with a different syntax (https://chromestatus.com/feature/6537562418053120), . Intent to deprecate old syntax (https://chromestatus.com/feature/5140610730426368) and shipping new syntax in 122 (https://chromestatus.com/feature/5586433790443520).

  - WebKit: Shipped in Safari 17.4 (https://developer.apple.com/documentation/safari-release-notes/safari-17_4-release-notes)

web-platform-tests: Tests available at https://wpt.fyi/results/custom-elements/state and https://wpt.fyi/results/css/selectors/parsing/parse-state.html.

Gecko-only tests were introduced: N/A

I am aiming to turn on CustomStateSet on by default by 2024-04-05, after a two week period of it being enabled in nightly builds. (Bug to turn on in nightly: https://bugzilla.mozilla.org/show_bug.cgi?id=1887467)

There is one small known bug with a patch ready to land (https://bugzilla.mozilla.org/show_bug.cgi?id=1887338), which I will land before turning on for nightly. Following that there are some small optimization tasks but nothing that prevents this from shipping, in my opinion.