Seems youtube.com did some update that is making HTML5 video playback stop working with a loud squeak and no play, or dropping to Flash. I'm experiencing it myself.

Submitted link to YouTube Google Product Forums.

From initial post: About an hour ago, all HTML5 playback just stopped. I was running OS X 10.10.1 (now 10.10.2) and Safari 8.0.

Videos would either switch to the Flash Player, throw an error, make a loud "squeak" noise and throw an error, or just crash the Safari page outright and forcing a Force Reload.

I know this is unrelated to the recent Yosemite update, as this issue was happening before I installed the update. In the meanwhile, Chrome does work.