When using GPU accelerated chromium based browsers in windows specifically Nvidia and Intel (AMD works for me), the 4:4:4 color mode does not work. This does not seem to occur on a linux client.
If you really need 4:4:4 color mode in your client you can pass --disable-gpu
to the browser launch command. The easiest way to do this on Windows is to modify a shortcut:
Create a new second shortcut for your chromium based web browser (edge chrome brave etc)
Right click it and select properties
Modify the Target to have --disable-gpu
at the end of it.
I understand this is not a great solution but honestly h.264 decode is super low cpu usage it is more watts on my machine to do it with my GPU. This is the only workaround I have found you have to bypass the proprietary decoder and use software decoding.