[webcomponents] Define the behavior of '::slotted' (the former name is '::content'). (#331)

Let's define the behavior of `::slotted`, a tentative name.
I am aware that CSS Scoping should define it, but let's have a rough consensus here.

I remember that I had discussion, maybe in www-style?, that we should make `::content` take only a simple selector, instead of a selector. Polymer guys told us that styling a *direct child* of an insertion point is the only use case for `::content` for them.

e.g.
::content(div)

That should be interpreted as `::content > div` in the previous syntax.

We no longer support a selector, like `::content(div p)`

We might want to adapt this restriction in `::slotted` because this makes the implementation simpler.

See also https://github.com/w3c/webcomponents/issues/308#issuecomment-143655347 because the definition will depend on these changes.


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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/331