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

Good question. I've never thought this case. :)

> ::slotted(div.foo::before)

This should not behave *as intended*, IMO. We might not want to support this because a pseudo element, which "::before" *matches*, is not a member of a distributed nodes of SLOT. This should not match. 

>  or ::slotted(div.foo)::before ?

How to support this kind of consecutive pseudo elements is up to each pseudo element. In this case, it's up to "::slotted" pseudo element.

In a general rule, selectors have a limitation about what can be followed by a pseudo element.
I'm +1 not to support this until we have a strong use case of this.

In a general rule, the support is poor

> Note: Note that, unless otherwise specified in a future specification, pseudo-classes other than the user action pseudo-classes are not valid when compounded to a pseudo-element; so, for example, ::before:first-child is an invalid selector.



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