https://html.spec.whatwg.org/multipage/semantics-other.html#pseudo-classes:popover-showing-state

Right now, popovers get a pseudo-class of :open when they're open, and :closed when they're closed.

These seem like really general names. Does it make sense for popover to own them?

Without understanding this is purely a popover feature, it seems weird that el.matches(':open') doesn't apply to open <details>, <dialog>, <select>, etc etc.