Andrew Kolakowski wrote:Yes, but in case of youtube you need many profiles at different resolutions, so when frame size changes importance of the same codec also weakens (no?).
yes, andrew, that's indeed a sound argument. there are many aspects, where this actual changes will have more impact than the benefits of using the same compression technology. but for other aspects, this doesn't have to be true. advanced features like the motion compensation capabilities or intra prediction features of newer codecs, which set them apart from more primitive predecessors, may be not affected by this changes to the same extent. the set of this kind of elementary compression features simple define the "language", how an indirect and more compact description of the image sequence can be expressed. and this also structures their capabilities to "translate" and prevent some aspects resp. image details across the mentioned changes.
in usage scenarios, where most of this circumstances remain unchanged (i.e. only intra frames, constant resolution and macro block size etc.), this doesn't have much significance. that's one of the reasons, why relative simple codecs, like prores/dnxhr/mpeg2, still behave exceptionally well in practical video editing work and usually do not show excessive generation loss. but the huge changes between the representation in mezzanine codecs and the final delivery and also between different types of highly compressed video footage are much more critical in this regard.
nevertheless your objection is perfectly valid and plausible, and i don't want to argue in a pure speculative manner. but i also try to consider this simple fundamental principles of data compression in my practical decisions. once you become aware of this kind of issues, you simply can not act anymore like a naive virgin and mix up all kind of completely different codecs in a carefree eclectic and arbitrary manner.
Andrew Kolakowski wrote:On top of this add fact that youtube uses low bitrates, so this will overshadow basically everything else.
yes -- but as already quoted in my last message, it still makes a difference, whether different codecs are in involved!
if you use the same or very similar compression method twice in different intensity, the result is indeed defined mostly by the stronger compression process. but in case of different compression methods, the errors introduced by both techniques may summarize.
Andrew Kolakowski wrote:Other way- it's way easier to "compensate" h264 encode by using bit higher bitrate than encoding VP9. It's waste of time to encode VP9 only because for given size you MAY get 1dB higher PSNR on youtube, which will look quite crap anyway.
well -- if you don't care about this minor differences, you could also just follow the advises for the average user at the youtube FAQ page.
and if you really want better results, self hosting is perhaps the most promising and more consequent way to archive this goal. but then, at the latest, you will have to learn a little bit more about the real benefits and practical relevance of vp9 in nowadays delivery of video data.

Andrew Kolakowski wrote:Also- don't forget that youtube still creates h264 encodes as well, so what you watch is not always VP9.
yes -- but that's more ore less just a fallback alternative of even less satisfying image quality for a nearly negligible small share of apple devices, which still do not support vp9. but in practice vp9 is by far the most important codec for video delivery over the web today. it's a little bit different, whenever hardware decoders and consumer devices are also involved, which often lag behind because of less flexible adaptation capabilities, but wherever software or more recent graphics accelerators are utilized for decoding, vp9 has undoubtedly won the game. h.265/hevc, which could have been an equal alternative, is simply irrelevant in practice, because it's not supported by most browsers due to all this well known licensing troubles.