After updating to Vivaldi 1.4 from the 1.3 version, for some –not all affected, only a few with particular hardware configuration are– HTML5 videos on YouTube and other sites have stopped playing, if you’re one of those, try the steps mentioned below to resolve the issue.
Vivaldi 1.4 HTML5 videos not playing
1. Visit vivaldi://chrome/settings/search#hardware and make sure hardware acceleration is enabled.
2. Visit vivaldi://flags and enable override software rendering list and restart the browser.
Now try to play any HTML5 video, it will play without any issues.