Add the media element pseudo-classes

[annevk]

These have been in Selectors for a while, but require host integration. The usage of public API in the definition feels justified given the current state of the media element section, though I tried to minimize it.

Test coverage can be found here:

• https://wpt.fyi/css/selectors/media/media-loading-state.html
• https://wpt.fyi/css/selectors/media/media-playback-state.html
• https://wpt.fyi/css/selectors/media/sound-state.html
• https://wpt.fyi/results/css/selectors/invalidation/media-pseudo-classes-in-has.html
• https://wpt.fyi/results/css/selectors/invalidation/media-loading-pseudo-classes-in-has.html

• At least two implementers are interested (and none opposed):
  • WebKit
  • Gecko
• Tests are written and can be reviewed and commented upon at:
  • See commit message.
• Implementation bugs are filed:
  • Chromium: https://bugs.chromium.org/p/chromium/issues/detail?id=1383480
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1707584
  • WebKit: N/A
• MDN issue is filed: Mention HTML Standard in media element pseudo-classes mdn/content#31746
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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/media.html ( diff )
/semantics-other.html ( diff )

[annevk]

@whatwg/media care to review?

[hober]

looks good to me.

[domenic]

Editorially LGTM; not sure what the current implementation status of these is.

[annevk]

@padenot you/Gecko still happy with these?

@foolip Chromium?

Still seems good to determine implementer interest as I guess in theory a host-dependent pseudo-class being standardized in Selectors doesn't mean anything if there's no host that does anything with it.

[padenot]

Mozilla position: this is a good initial set.