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

> 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).

Fully isolated components are a trivial case in the vars/apply/nesting feature set; you just **don't use any undefined variables**.  I presume this is the same as in ::part - just don't expose any parts.

If you further want to block sub-components from receiving styling, this is trivial in vars/apply/nesting, as I demonstrated above (or if you want to block *everything immediately*, for all your sub-components, just apply a `:host { --: initial; }`, or maybe just rely on the "no inheritance by default/ever" concept that I assume isolated components will have).  

Since we have no idea how ::part will expose/hide the parts of sub-components, we can't make a proper comparison yet, but we can at least see that the v/a/n option is trivial.

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