[selectors] Add CanIUse panels

[cvrebert]

Bikeshed has a feature to include CanIUse info panels in specs.

Based on an analysis of CanIUse's database, Selectors is the CSS spec with the most CanIUse data, and that data is fine-grained. And Selectors is a fairly fundamental spec. So I'd like to suggest adding CanIUse panels to the spec.

Here are the spec anchors and corresponding CanIUse features for Selectors:

Spec URL	CanIUse URL
https://drafts.csswg.org/selectors/#attribute-case	http://caniuse.com/#feat=css-case-insensitive
https://drafts.csswg.org/selectors/#descendant-combinators	http://caniuse.com/#feat=css-descendant-gtgt
https://drafts.csswg.org/selectors/#first-line	http://caniuse.com/#feat=css-first-line
https://drafts.csswg.org/selectors/#indeterminate	http://caniuse.com/#feat=css-indeterminate-pseudo
https://drafts.csswg.org/selectors/#matches	http://caniuse.com/#feat=css-matches-pseudo
https://drafts.csswg.org/selectors/#negation	http://caniuse.com/#feat=css-not-sel-list
https://drafts.csswg.org/selectors/#optional-pseudo	http://caniuse.com/#feat=css-optional-pseudo
https://drafts.csswg.org/selectors/#placeholder	http://caniuse.com/#feat=css-placeholder-shown
https://drafts.csswg.org/selectors/#range-pseudos	http://caniuse.com/#feat=css-in-out-of-range
https://drafts.csswg.org/selectors/#relational	http://caniuse.com/#feat=css-has
https://drafts.csswg.org/selectors/#the-any-link-pseudo	http://caniuse.com/#feat=css-any-link
https://drafts.csswg.org/selectors/#the-default-pseudo	http://caniuse.com/#feat=css-default-pseudo
https://drafts.csswg.org/selectors/#the-dir-pseudo	http://caniuse.com/#feat=css-dir-pseudo
https://drafts.csswg.org/selectors/#the-focus-within-pseudo	http://caniuse.com/#feat=css-focus-within
https://drafts.csswg.org/selectors/#the-nth-child-pseudo	http://caniuse.com/#feat=css-nth-child-of

[svgeesus]

@tabatkins is it still planned to add CanIUse to Selectors 4? Is this a nice-to-have or essential?

[castastrophe]

@tabatkins Is this a feature I can assist with implementing?

[tabatkins]

@castastrophe Sure, feel free to do so. https://speced.github.io/bikeshed/#metadata-include-can-i-use-panels

[cvrebert]

IMHO, this has become moot due to MDN largely superseding CanIUse. I've filed #14107 as the one selectors item missing an MDN panel from this issue's original list.