[youtube] `maxresdefault` doesn't exist sometimes

Originally posted by @Zirro here

I’ve received several 404’s for maxresdefault URLs with this change, so unfortunately it doesn’t seem to be true for every video. The best approach would probably be to initially request maxresdefault and look at the response code. If it’s 404, try with hqdefault instead.

It could also be worth using a similar approach to check for the existence of a .webp-version of the thumbnail at the /vi_webp/-path before falling back to the .jpgˋ-version, though the difference in quality doesn’t seem to be very significant here.

Here’s one where there is no maxresdefault size at all: https://www.youtube.com/watch?v=pS4ca4GB-_c

https://i.ytimg.com/vi/pS4ca4GB-_c/maxresdefault.jpg (404) https://i.ytimg.com/vi_webp/pS4ca4GB-_c/maxresdefault.webp (404)

Here’s one where both JPEG and WebP-files are accessible at separate URLs: https://www.youtube.com/watch?v=goSEyGNfiPM

https://i.ytimg.com/vi/goSEyGNfiPM/maxresdefault.jpg (200) https://i.ytimg.com/vi_webp/goSEyGNfiPM/maxresdefault.webp (200)

Here’s one where only the JPEG-file is available: https://www.youtube.com/watch?v=xp-b4Ce4Mf4

https://i.ytimg.com/vi/xp-b4Ce4Mf4/maxresdefault.jpg (200) https://i.ytimg.com/vi_webp/xp-b4Ce4Mf4/maxresdefault.webp (404)

I’m not sure if there are cases where only the WebP exists, though. Besides that it’s mainly a question of whether checking for the .webp first should be done for the theoretical quality improvement, with the .jpg staying as fallback when there is no WebP available.