[webcomponents] Clarification needed :root CSS pseudo class matches ShadowRoot or not (#338)

As currently specced, CSS pseudo class `:root`

https://drafts.csswg.org/selectors-4/#root-pseudo

> The `:root` pseudo-class represents an element that is the root of the document.
> For example, in a DOM document, the `:root` pseudo-class matches the root element of the Document > object. In HTML, this would be the html element (unless scripting has been used to modify the document).

which looks unclear whether in a shadow tree it matches to its shadow root or not.
Note that I understand shadow root is a DocumentFragment (if #325 is settled down) and
is not an element, thus normally a CSS selector cannot point to it.

If `:root` is used inside shadow style, the selector never matches anything?

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/338