Cannot play live Youtube on other streams in Opera - no h264 support

I have good reasons to use Opera as my default browser (I find it lighter than Firefox Quantum and Chrome/Chromium, I like its speed-dial bookmarks, as well as other features and details). Opera browser cannot play Youtube streaming videos . **Normal (non-live) youtube videos work fine.** **I know Youtube doesn't involve flash**, but in order to debug this in a larger context it may prove usefull to say that the same browsers that do not play live streams in Youtube (**Opera and Chromium**) are also affected by a problem where they cannot play (at least) some other (probably flash) internet video streams - while Google Chrome and Firefox are not affected by neither of these problems. Help me debug this. I am in elementary OS Loki up-to-date, Opera 52.0.2871.40. I have also posted this question: [How to tell if an internet video is flash or html5 ](https://superuser.com/q/1314426/162573) . --- Edit --- Installing chromium-codecs-ffmpeg-extra for the purpose of playing Vimeo videos (as indicated [here](https://help.vimeo.com/hc/en-us/articles/115015677227-Troubleshoot-player-error-messages)) solved **for Chromium** all the problems I had with both Opera and Chromium (not playing live streams and a lot of other non-live online video): but **not for Opera**.