Problem with H.264 video. It works differently in different browsers within the same computer, how to solve the problem?

Question

Opera

After the announcement about the termination of support for the flash player by top browsers, there was a problem with video playback on sites. I've been suffering for a year now. The video is literally in a fever on YouTube and other sites: either it doesn’t play at all, or it plays in low quality, or in general the problem recedes and everything works.
The main browser is Opera. I reset the settings, looked for the necessary flags in browsers, googled - but did not find a solution ...
The most interesting thing in google chrome is always everything is in order. I quote from the screen of this strange behavior from the test on YouTube.

Answer 1

Well, it doesn’t support a specific codec, maybe it’s worth putting it into the system?
Well, or in the old fashioned way, all codecs at once https://www.codecguide.com/download_kl.htm