I have explicitly opted out of the trial:
However about 10% of videos still play in HTML5.
I found this question however that tells you how to do it in firefox and not chrome.
on 09/15/2013 – Made popular on 09/15/2013
Sometime over this weekend--I think after an update--I started noticing that videos on Youtube in Chrome have spots on them, and that's with both Flash videos and HTML5 videos. This doesn't happen on other video websites, or even in Firefox.
Alright, this is weird. Every time I try to watch live YouTube videos on the YouTube website, I get the "TV static" error screen with the message "Your browser does not currently recognize any of the video formats available.