Just to clarify something here: I too had no problems viewing YouTube videos after installing the nvidia proprietary video drivers, but when I tried to view streaming videos from other websites, either I got an error message saying that I didn't have the correct codecs installed, or the vvvideo in question would play back in super-slow motion (with no sound).

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And you mean that those other websites work fine in Firefox if you do not install the binary nvidia legacy driver and use the opensource nouveau driver instead? My guess is that you with the nouveau driver do not use some hardware accelerated video playback feature that the binary nvidia driver tries to use.

The binary nvidia driver does provide some hardware accelerated video playback capabilities, but those capabilities probably only work for a selected set of video codecs. Maybe you have stumbled upon some codec which looks like something that should be supported by hardware accelerated video playback, but does not work very well with it.

During the years, there have been new video codecs coming like mjpeg, mpeg, h264... During those years support for new codecs has been added to software and in some cases in hardware. However, if you want to play the latest video codecs with hardware acceleration you might also need the latest hardware.

regards Henrik

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Don't confuse two completely different things. There is hardware decoding [of video] and there is hardware rendering. First works reasonably well and resorts to software decoding when given hardware does not support the codec used. Rendering with hardware acceleration has always been problematic, this is where your problem usually is. For instance my mpv.conf, in my Intel box:
Decoding is set to VA-API, hardware decoding.
Video out is set to gpu, when I set it to vaapi (hardware rendering) then mpv warns me it is not the best idea, but works.

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