Check out also the overview of Vivaldi: https://mxlinux.org/wiki/applications/vivaldi/
As Vivaldi comes OOTB with support for open codecs only, propriatary media codecs (MP4[H.264/AAC]) need to get installed manually by the user. Vivaldi has an inbuilt test to check for available ffmpeg library in the system: open a terminal an run Vivaldi from the command line: vivaldi If the missing propriatary codecs have been detected you'll see something like:
No suitable library for HTML5 proprietary media (MP4[H.264/AAC]) was found, therefore only open codecs will play. To add support for proprietary media, issue the following command and restart Vivaldi: curl https://launchpadlibrarian.net/414953672/chromium-codecs-ffmpeg-extra_73.0.3683.75-0ubuntu0.16.04.1_amd64.deb |\ tail -c+1077 | tar JxC ~ --wildcards \*libffmpeg.so --xform 's,.*/,.local/lib/vivaldi/,'
Copy the line starting with curl and the line below (that’s actually one command line), paste it into a terminal and hit return.
To Check support of HTML5 proprietary media (MP4[H.264/AAC]) you can open this side: https://www.quirksmode.org/html5/tests/video.html