Regarding youtube video formats... As far as I gather there are h264, vp9 and dav1d. vp9 seems 'standard' but does YT provide any video in any of these formats? Will this shift in the future?

On PPC Linux vp9 is the most problematic. Quite some endian issues and it does net pass the testsuite at all on ppc. dav1d passed it's testsuite with ease on my G4 but is rather slow on the other hand, lacking Altivec optimizations. h264 fares quite well on ppc/x86/x86_64 but has lowest quality.