It's not actually, enabled. It says that it is, but it's really not. It's still using your CPU to decode 4K video.

https://www.phoronix.com/scan.php?page=news_item&px=Chrome-N...

https://bugzilla.mozilla.org/show_bug.cgi?id=1210726

https://wiki.archlinux.org/index.php/Hardware_video_accelera...