Add support for CSS Scoping module

[strarsis]

https://www.w3.org/TR/css-scoping-1/
The deep combinator seems to be often used in SVGs.

Currently these deep combinator selectors cause a SyntaxError in underlying css-what.

[fb55]

Not sure about issue 6:

This is basically a super-descendant combinator. If the descendant combinator had a real glyph, it would potentially be interesting to just double it. Maybe we can give the descendant combinator a pseudonym of >>, as it itself is a super-child combinator? Then /deep/ could be spelled >>>

Does that mean that the syntax is about to be changed? Should we implement this right now?

:scope is already supported, the deep combinator seems a bit harder. Besides the syntax question, as there is no implementation of something shadow DOM-alike, what would the semantics be? Just continue as if it wasn't there?

[strarsis]

For shadow DOM support, the existing API would need additional optional methods like getShadowRootElement().

[fb55]

Sorry for leaving this hanging. Having this in the adapter seems like a good idea, PRs welcome :)

[strarsis]

https://www.chromestatus.com/features/6750456638341120 😕

[fb55]

That's sad. I guess if there isn't a replacement this can be closed?