On the other hand, pretty easy how such a div might trigger a less efficient path; if the video is top in the z-order then it can probably bypass being composited by the browser (and who knows, maybe even bypass being composited by the OS) and avoid a whole mess of rendering to a texture, texturing some triangles, and so on and so forth.
Right. My take is "hey, good engineering to not need a power brick. But to what end? What's the practical purpose of this engineering? Why not just make the system bigger?"
One of the (many) reasons I've heard they had to abandon Longhorn was that apparently it was essentially unbuildable. The build was broken so much of the time that they gave up and started over.