Comment by pavpanchekha

I think by "JavaScript" here you mean rendering—that's partially true. In macOS and Windows these days (also I think Linux with GTK4 on Wayland, though only in a limited way), the window manager is itself composited and a window can send a small display list to that window manager for it to composite. In that case, it's possible to actually have the video decoding to happen entirely in hardware and never have the browser directly interact with decoded video bits. That said usually the window manager compositor is pretty limited and the browser will only do this when the stars align. The sort of things that can break it are any kind of weird clipping, transparency, or effects applied to the videos.