YouTube is undergoing some experimental layout and viewing changes as part of a test project called Cosmic Panda. Click that link to partake in the latest TestTube project. Some of the changes will look familiar, like the playlist view that takes some inspiration from Leanback. Others are brand new, and focus on a modern, wide view.
You can enable the new features at the Cosmic Panda info page and return to the old YouTube at any time. Don’t forget to submit feedback!
Via YouTube blog