I'd guess it's not be about the codecs, but about hardware acceleration. I haven't tried Xbox Cloud Gaming, but when I tried Stadia a couple of years back it wa

I'd guess it's not be about the codecs, but about hardware acceleration. I haven't tried Xbox Cloud Gaming, but when I tried Stadia a couple of years back it was completely unplayable with software VP9 at 1080p vs. as good as playing locally with hardware decoding. The game streaming use case seems to be very different from general purpose video streaming: it's realtime, is not efficiently compressed (due to the realtime nature + tight latency budgets during encoding), and changes rapidly unlike the main other use of realtime video streaming (video calls). So if software decoding is fine for watching Netflix or making a Zoom call, this is different. Browsers have APIs for detecting codec support, but I don't think there's anything for detecting hardware accelerated video decoding. But given Chrome on Linux doesn't support this on any hardware, it's a pretty decent heuristic.