[css-pseudo] ::file-selector-button as a CSSPseudoElement #14547

In #12251 we made the ::file-selector-button be a tree-abiding pseudo element, which previously was an element-backed pseudo element.

CSSPseudoElement says that it represents any tree-abiding pseudo element that isn't element-backed.

Combining the two, it means that ::file-selector-button should be representable as a CSSPseudoElement. Now, the problem is that in practice (in at least Blink), ::file-selector-button is implemented as a shadow tree with a different element backing it, so it's technically implemented as element backed and exposing it as CSSPseudoElement seems potentially problematic. That is, you'd have some indirect reference to a UA tree scope element. Right now, there's a limited number of things you can do with that, but it's still different from most other tree-abiding non-element backed pseudos. (Again, in Blink, ::placeholder also has the same problem)

I'd like to discuss. I personally would say that ::file-selector-button should be element backed, but (IIUC) that causes interop problems. It's just the CSSPseudoElement adds a layer of complexity here if we want to support ::file-selector-button as tree-abiding non-element backed. Either that, or we special case it as not being supported in CSSPseudoElement.