Vivaldi doesn’t play videos.
Bug report in Gentoo.
Here is the fix:
mv /opt/vivaldi/lib/libffmpeg.so /opt/vivaldi/lib/libffmpeg.so.old
ln -s /usr/lib64/chromium/libffmpeg.so /opt/vivaldi/lib/
I don’t like to delete files, so I just rename them. That way I can put everything back the way it was if the ‘fix’ doesn’t work.