https://github.com/popcornmix/omxplayer and Kodi are the only ones I've found that handle it correctly (and those are related/share code) - that means Kodi is the only place to play 1080p or larger youtube video (omxplayer can stream video from youtube, but doesn't by itself handle the separate audio and video streams).
Re:Designing Raspberry Pi 400
by entropicdrifter on Wednesday November 04, 2020 @12:25PM
(#24991050)
Looks like there is a way to enable hardware decoding in Chromium, at least on Raspberry Pi OS:
https://lemariva.com/blog/rss/raspberry-pi-4-video-accelerat...
However the fine print looks to be that this may not hold true for Chrome and streaming / YouTube, but offline videos with certain players because Chromium doesn't support hardware acceleration for this API. Chromium supports VAAPI, this uses libva. sigh Why does this Linux hardware decoding story always repeat itself. :-(
Still, raw CPU may let it manage at least 1080p at 30 fps but I can't say for sure for anything more.