> The video decode itself is accelerated, but each frame passes through JavaScript to be composited

I don't think that's true, and it's even less true once DRM video is involved - it becomes very difficult to get other software on the machine to even see the video, at least on Windows. You can very occasionally see bugs where the hardware accelerated playback ends up in a different place to where the browser thinks the video should have been put, too.

What does happen is the video data gets reassembled in Javascript (e.g. Video.js) because the native player doesn't support HLS. Not quite the same thing. It's just reformatting MPEG-TS to the similar but not identical MP4. Oddly, the browser in my LG TV does play HLS video natively, and I think Safari does?