It appears as if only some videos on YouTube will play using HTML5 and without Flash. For example, this video plays just fine for me and I do not have flash installed: http://www.youtube.com/watch?v=fxEWddT44BY
So, unfortunately, you will have to install flash or wait for YouTube to fix their website.
Also, one thing to try: When I had Flash installed and the "ClickToPlugin" Safari extension installed, YouTube would load all the movies in a simple HTML5 format. So no annotations or any other annoying stuff. Might be worth trying. You will have to install flash but, with the "ClickToPlugin" extension, you don't have to use Flash.