I wanted to give the button a different style if its associated popover is shown. There is no pseudo-class for button:pressed...There is, however, a pseudo-class to tell you if a popover is open: :popover-open.
Behold the almighty God Selector: :has(nav:popover-open) button[popovertarget] {background-colour:goldenrod;}. This is omnipotent because it doesn't require any structural relationship between the thing being checked and the thing being styled.
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