Hardware Video Acceleration is still touch and go on Linux in general (especially on ARM). Things have gotten a lot better with dedicated video applications but browsers are a whole 'nother ball of wax. Neither Firefox nor Chromium support it in Linux. The latter even has a special build in some distros to enable it. Check out this entry in the Arch Wiki about Application Support.
To help with this issue, Mladen wrote this PureOS Wiki entry to allow web videos to be opened in GNOME MPV from PureBrowser. its a lot less resource intensive doing so through MPV than a browser, even at 720p+.