From this 
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The lastest YouTube’s testTube experiment is now released and you can try it by going to www.youtube.com/cosmicpanda. Google seem to have gone design crazy and have been redesigning some of their apps recently, and now they did the same to YouTube with the help of Cosmic Panda, not only is the new design sleek, it’s easier to find menu and easier to use. Not too sure about the name and the logic behind it, but who cares, it shoes Google are fun.
According to zdnet.com Chrome users will be treated to even more features, including the ability to maintain video playback while toggling between videos, playlists, and channels.
If you try it out and dislike it, you can revert back to the old style.
