I binned Opera due to this, if they ever fix is issue I might go back to using it.
But my confidence in the Devs is low, it's a bad reflection on Opera.

Just adding my voice to this matter as I have been Googling(.com) as to why I can't have google.co.uk as my default search engine.
Simple things like shopping where I want to limit my search results to UK sites only are now defunct. I don't want international results forced on me.
This is a massive oversight by the developers.

I came here with the same problem - a quick search has fixed it for me - disable the Opera Adblocker.
Menu > Settings > Block ads

However, the HTML5 video test you linked to worked for me with the adblocker enabled, so I'm unsure if we have the same problem.