[css-ui] Add CanIUse panels

[cvrebert]

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

I'm wondering if CSS Basic UI would consider using this feature? Here are the spec anchors and corresponding CanIUse features for CSS Basic UI (ignoring the cursor features due to ambiguity):

Spec URL	CanIUse URL
https://drafts.csswg.org/css-ui-4/#appearance-switching	http://caniuse.com/#feat=css-appearance
https://drafts.csswg.org/css-ui/#box-sizing	http://caniuse.com/#feat=css3-boxsizing
https://drafts.csswg.org/css-ui/#resize	http://caniuse.com/#feat=css-resize
https://drafts.csswg.org/css-ui-4/#text-overflow	http://caniuse.com/#feat=text-overflow
https://drafts.csswg.org/css-ui-4/#valdef-user-select-none	http://caniuse.com/#feat=user-select-none

[frivoal]

Seems reasonable. I don't think I've seen it much in other css specs, but unless someone comes up with a good reason why doing this is bad somehow, I'll probably do so.

[LeaVerou]

Huge thumbs up for this! That would be fantastic!