This work was done by Martin Stránský from Red Hat, as with most of the other linux hardware acceleration and Wayland enablement work that has happened over the past several years.
But Red Hat is not very popular right now, so even mentioning that they might be due credit for this work is therefore considered potential "rage bait".
I very much don't want to see the thread devolve into another argument either, but I also hope that can be avoided without completely ignoring such details as the engineer that did this work (and lots of other work) in Firefox.
Disclosure: I work at Red Hat
I support open source, I believe it needs to be sustainable financially otherwise it will just be replaced by closed source competitors and the world will be better off.
Besides, I'm still going to play videos in mpv anyway. In-browser players suck for various other reasons besides the acceleration/performance issue. The bare-bones vanilla in-browser player has virtually no features and controls cannot easily be remapped. Firefox's floating video player feature is a nice step in the right direction, but only a tiny step. Furthermore the video controls implemented by websites like youtube are even worse and fixing that on a per-site basis would be a huge chore. I'll give you a specific example: my laptop's universal volume keys are my F1, F2, and F3 keys and they're right next to my 1, 2, and 3 keys. In the control scheme youtube has implemented, if I ever miss my volume keys I jump the video back to 1:00, 2:00 or 3:00 because youtube for whatever reason thinks number keys should be shortcuts to seek to that minute mark. I guess some people probably like this but I certainly do not. I could try to fix this by injecting scripts into the youtube page, but that's a hassle I can simply avoid in the first place by using a video player that has mostly sane defaults and makes configuration straight forward besides. I'd rather configure mpv once then play wack-a-mole fixing numerous websites and keeping those fixes up-to-date and working.