I started using Vivaldi today (I can't cope with Google's 'my first browser' that Chrome has become due to Google's lack of visual attention, not to mention the insanely backward capability and visuals of the youtube app aaaarrggh!!) and I hit a problem - some youtube videos don't work and when I used this page:
it said I didn't have H.264 support and it seems that this is due to licensing.
I contacted Josh Strobel on twitter and he pointed me to the following:
and it works now, if I look at the same youtube test page it says H264 is now supported.
Josh also said that this is only necessary until Chromium is in the repo and Solus can use it's free codec for Vivaldi.