Re: [w3c/webcomponents] Custom pseudo-classes for host elements via shadow roots (:state) (#738)

Possible tangent: The `:heading(n)` pseudo-class is not a state in this sense (unlike `:hover`, `:active`, `:checked` and things like that.) While some CSS pseudo-classes represent internal states, others, such as `:nth-child`, `:matches` and `:empty` don't represent internal states, but rather express conditions about the structure of the DOM. `:heading(n)` seems like it belongs more in the latter category.

Similarly, not all pseudo-elements are parts, so for example `::part` should not be extended to cover cover `::selection` even if it addressed other kinds of built-in parts in a future extension.

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