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Currently there's no mention of custom pseudo-elements in the explainer. Seems like that would fit in section 6.3 CSS and Shadow DOM.
Relatedly, @host rules, ::distributed and ::shadow could be exhibited too.
Oops. Not yet.
I have made a first pass over this: <https://dvcs.w3.org/hg/webcomponents/rev/423e7d313148> ::shadow is not mentioned because it is not in the normative spec yet so I'd prefer to wait on that one. @host, ::distributed and pseudo-elements are explained, though.