Re: [w3c/webcomponents] Support Custom Pseudo-elements (#300)

Thanks for writing up a draft!

Early comments:
(1) Style nit: the motivation section goes on too much about past proposed solutions for this problem but that weren't adopted. It's probably better to have the motivation just state the use case, perhaps mentioning the analogous case of styling for built-in controls, for those browser that have it.

(2) It seems like ::theme() is only useful for open mode shadow DOMs that have not done appropriate forwarding for further shadow DOMs that they contain. Is that really a big enough use case? The forwarding feature seems like a cleaner way to handle this, and one that also works with closed-mode shadow DOMs.

(3) It should be possible for components to restrict the set of CSS properties that can be applied to a part, ideally via a whitelist. (a) Built-in elements that expose pieces as pseudo-elements all do this; and (b) it seems likely that some CSS properties could break components in unexpected ways.




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