Re: [webcomponents] CSS Cascading Order in Shadow DOM should define behavior for closed shadow trees (#293)

>  given that a selector defined in ancestor trees is never applied to a node in a closed shadow tree.

I meant that, I agree with what you wrote and I believe it is a common understanding among us, but I could not find definitions in relevant specs (Shadow DOM spec, CSS Scoping spec, nor in the @rune-opera's [proposal](https://github.com/w3c/webcomponents/blob/gh-pages/proposals/Shadow-DOM-Cascade-Order.md)).

I suppose there are several edge cases that I think it's worth to clarify, such as:

1. Closed shadow tree has closed child shadow tree. Should we prevent cascading? Probably yes?
2. Closed shadow tree has open child shadow tree. Should we cascade? Probably yes?
3. Closed shadow tree has open child shadow tree, and the child uses `:host`. Should this work, or prevented?
4. A closed shadow tree has open child shadow tree, and the child shadow tree used `::content`, in which case the distributed content could be in a closed tree. I guess it should work?

Sorry that I haven't given a full thought yet and dumping as I think, but these look a bit complicated, no?

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