MP4NoMore

This extension helps, but does not fix the issue with Vivaldi, regarding the inability to render MP4 while being able to with other browsers.

Q&A

Q. I still get "The media could not be played" on some sites.

A. Consider in most cases that the site is only offering MP4. If the site only offers MP4 and this extension doesn't allow us to be served mp4 due to the issue, you will still get that error message. I have found this to be common with some Twitter Video, for example