Re: [webcomponents] Support Custom Pseudo-elements (#300)

Like we agreed during the last F2F, custom properties, `@apply`, and custom pseudo elements aren't conflicting ideas.  They address slightly different use cases and both can be supported.

In particular, in an isolated web component where neither component nor its container document trusts each other, we can't expose all mixins defined in the container document into the shadow tree.

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https://github.com/w3c/webcomponents/issues/300#issuecomment-143954254