My understanding is that most video websites like Youtube use Media Source Extensions, a standard which allows javascript to download and assemble video streams

My understanding is that most video websites like Youtube use Media Source Extensions, a standard which allows javascript to download and assemble video streams in non-standard ways. This work is done by page JS and passed to the browser to decode and render. AFAIK youtube uses this to implement DASH streaming. Presumably there is no easy way to disable page JS but keep your youtube video downloading.