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Huh? Why does the empty HTML element do that? YouTube has a lot of not-empty HTML elements overlaid on the video too... I can't imagine a simple <div> alone is doing this, otherwise wouldn't the rest of the UI also do that?

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This is standard practice, the system compositor has a fast path for video decoding and rendering. The fast path will not be taken when software composition is needed (eg, something is overlaid or blended).

If you’re asking why they didn’t think to design the fast path to deal with Byzantine faults, it’s probably because why the hell should they?

Yes, I understand that. However, empty divs are not an edge case, and I can imagine a lot of not-insane reasons an empty div might be on top of a <video>. The entire player chrome is within the video nowadays, and it often intersected in the past (the old seek bar for example.) If there is more to this story than an empty div, I would be interested to hear. Today's YouTube seems to put display: none on almost all of the overlays when the user is not hovered, and if that isn't good enough, I think that is a bit absurd. (Actually, there are some empty divs, but they're really just empty... Some contain display: none'd divs.)

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