[CSS] Selectors, Fx 77 tasks

[chrisdavidmills]

Trello

https://trello.com/c/ZHrUvxzS/29-selectors-fx-77

Acceptance criteria

1. • Implement :is() and :where() selectors (see also intent to ship; also, this may be behind a flag for now; see below bug)
2. • Enable :is() / :where() on all release channels.

[chrisdavidmills]

Actions taken here:

• :where() page completed — https://wiki.developer.mozilla.org/en-US/docs/Web/CSS/:where. Example added, and page cleaned up.
• :is() page updated, to link to example on :where() that explains the difference: https://wiki.developer.mozilla.org/en-US/docs/Web/CSS/:is#Difference_between_is_and_where
• Entry added to Experimental features pages: https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Experimental_features#CSS
• PR added, for BCD update: updating firefox support data for :is(); adding support data for :where() browser-compat-data#6179
• No rel note for now, as it is not enabled in release yet.

[Elchi3]

Thanks Chris!
We will have a new story on this for https://bugzilla.mozilla.org/show_bug.cgi?id=1632646 so that we can talk about it in the Firefox 78 cycle.