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

[Cascade Order for CSS rules from different shadow trees](https://github.com/w3c/webcomponents/blob/gh-pages/proposals/Shadow-DOM-Cascade-Order.md) should be aware whether a tree is closed or not, and probably define different rules if closed.

Pointed out by @rniwa in an off-list conversation.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/293