I know that HEVC isn’t a file format, but I can’t comprehend all this. Is it the buffering? Size? Video player’s behaviour?
Can I rename the HEVC.mkv to HEVC.heif and let the browser play it like a GIF? And the same for AVIF vs AV1? Did not have time to read all the docs.
It really depends on what you use. There are tools that will extract the different data streams inside the containers and you could add them to a different (compatible) container… However if you use some sort of converter tool, it may just read the data stream, decompress it and decompress it again which would under the best circumstances lose quality to it’s size.