RE: Opera Version:58.0.3135.117 Opera is up to date but Videos don't play
I don't get you...
Regardless of browser and html5 capabilities/implementations, do you want to use webm on youtube or not?
This addon is an... enhanced version of h264ify and nothing more. All they do is block youtube's codec detection, and all is done via js. This way, youtube plays in whatever codec you "allow" it to play. You can check it in youtube.com/html5 .
On top of the above, and assuming a browser supports all the codecs youtube can use, youtube forces the usage of webm for the videos, because it uses less bandwidth for the same resolution.
However, unlike h264, webm is not hardware accelerated, and that is noticeable with its bugger cpu usage, which results in choppy playback on weaker machines, bigger battery consumption on laptops etc.
And all that is just theories for opera, since it does not support h264 by default.