popover currently fires a beforetoggle event an asynchronous tick before the toggle event. This is useful for a few use case (cancelling, kicking off work like animations). It would be nice for <details> to do the same, as <details> already has a toggle event.

During the triage meeting at TPAC, while discussing #9733 we looked into this and there are some constraints to consider: namely that it <details open during parse should not trigger a cancellable beforetoggle event. Otherwise it should be cancelable, like popover and (soon to be) <dialog>.