Re: [w3ctag/design-reviews] [WebComponents] Custom state pseudo class (#428)

I'd like to stress that this feature is "custom pseudo classes". It is not "general state exposure mechanism". It is not "expose arbitrary data structures for CSS selection". It is just giving custom elements the ability to do what built-in elements do: expose boolean bits of external state, in an element-controlled fashion (unlike class names), to their consumers.

There is an additional capability being discussed, which is the CSS "pseudo class functions" like :lang() or :dir(). I think that would be an additional interesting capability to explore allowing customization of. But, despite both starting with :, I agree with others who have stated that this will require a new, separate API, because it targets rather different underlying models and use cases. For example, something analogous to :lang() would likely not want to be tied to particular element states, but instead run a (JS?) function evaluating computed properties *all* elements, as the spec for :lang() does. I don't think such "custom pseudo class functions" would be tied to custom elements at all, most likely.

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