Huh, apparently some browsers can display 4:2:2 subsampled video? I'd appreciate some wider testing: https://www.kepstin.ca/dump/coco240p.mp4 (video description: a burst of static and some flashing frames, then a green background from an early-80s microcomputer showing boot messages and a flashing cursor)
@kepstin plays well here on firefox + linux, although it does buffer quite long (and multiple times)