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

@philipwalton : My point in [the comment](https://github.com/w3c/webcomponents/issues/300#issuecomment-144570887} is nothing to do with whitelisting properties but more to do with exposing mixins and properties across component boundaries.

In the case of custom pseudo elements, the authors of components are explicitly opting in the contract that a part of its component is stylable by its user, and the users of components are similarly opting in to style those parts without exposing any other mixin or custom property defined in the document.

This is a crucial property for the isolated components where neither component author nor component user trust each other (e.g. for cross-origin widgets).

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