Vivaldi ( and Opera ) won’t play online videos out of the box on Gnu/Linux. This is something that bugged me a few months back. Fortunately, the solution should be simple. Go to this link: https://github.com/iteufel/nwjs-ffmpeg-prebuilt/releases?after=0.32.1
and download ffmpeg.so file. I recommend trying version 0.30.5 first. Some of the newer ones didn’t work for me. Download and unpack it, then copy it to /usr/lib64/vivaldi under root privileges.
sudo cp libffmpeg.so /usr/lib64/vivaldi
Reopen Vivaldi, and problem should be fixed. The same applies to Opera, except that you should copy the file to /usr/lib65/opera.
Hope this helps