I think, it was last January when YouTube announced that they are testing HTML5 features on their videos. Finally they launched their stable HTML 5 player for YouTube videos .HTML 5 will be the new dimension in web format more over it getting huge popularity now. It will be going to add more features to your web experience.
You Tube finally push a stable HTML 5 player support for their videos. Now you don’t need Adobe flash player to view YouTube videos on the intenet.Except internet explorer most of the browsers supporting HTML 5 now. I hope IE new version will come with HTML 5 Support soon.
You will not feel any much difference in the HTML 5 YouTube video. The only difference is that, the video being rendered with out Flash Player.
You can watch a single YouTube video in HTML 5 player by adding a special parameter &html5=True right after your video address.
For example: http://www.youtube.com/watch?v=cTl3U6aSd2w&html5=True
If you have an HTML 5 supported browser then no need to add this special parameter.