[w3c/webcomponents] Shadow tree encapsulation and CSS (#804)

I'd like to open this as a tracking issue to figure out to what extent CSS and Selectors violate the encapsulation principles we agreed on and what can be done about it. If you know of any such issues please add them here.

A clear one to start with is `:focus-within`. If `:focus` matches and `:focus-within` does not, there's a shadow tree. (To be clear, the mere fact of exposing that there is a shadow tree violates encapsulation, as previously established.)

cc @emilio @tabatkins @fantasai

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