Re: [webcomponents] :unresolved vs :not(:upgraded) (#418)

Until they're upgraded, the elements are of type HTMLUnknownElement, so how about `:unknown`?

That describes their current state using existing terminology, avoids use of `:not`, and is a simple, natural word.

It also has the benefit of not presuming that the unknown element is going to be upgraded using the Custom Elements API. There are JS frameworks that make use of non-standard tags, and they could use `:unknown` as well.

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