Thanks for clearing up the CA stuff
> I believe, but am not sure, that H.264 is decoded in software in Firefox but is decoded in hardware in Safari.
I am 100% certain it is (was?) on Firefox as well, because it would have been impossible to hit the aforementioned 60 energy impact without hardware acceleration. This is done through VideoToolBox, but that might have changed.
I think CA was only moved to the window server relatively recently. Before then, my understanding is that it simply used OpenGL in-process, so it had no energy advantages over Firefox's built-in compositor. There was also a long time when the APIs to host CA content in the window server were private APIs and only Safari could use them.
> Also, coincidentally, do you know what exactly causes the H264 problem on macOS?
I believe, but am not sure, that H.264 is decoded in software in Firefox but is decoded in hardware in Safari.