It's another version of the center-aligned layout, which uses a persistent header, a guide sidebar that's visible only when you click "Guide" and a card-style interface.
Here's how you can enable the experimental feature. If you use Chrome, Firefox, Opera, Safari or Internet Explorer 8+:
1. open youtube.com in a new tab
2. load your browser's developer console:
* Chrome or Opera 15+ - press Ctrl+Shift+J for Windows/Linux/ChromeOS or Command-Option-J for Mac
* Firefox - press Ctrl+Shift+K for Windows/Linux or Command-Option-K for Mac
* Internet Explorer 8+ - press F12 and select the "Console" tab
* Safari 6 - if you haven't enabled the Develop menu, open Preferences from the Safari menu, go to the Advanced tab and check "Show Develop menu in menu bar". Close Preferences and then press Command-Option-C to show the console.
* Opera 12 - press Ctrl+Shift+I for Windows/Linux or Command-Option-I for Mac, then click "Console".
3. paste the following code which changes a YouTube cookie:
document.cookie="VISITOR_INFO1_LIVE=lnq3tYnrDNU; path=/; domain=.youtube.com";window.location.reload();4. press Enter and close the console.
To go back to the regular interface, use the same instructions, but replace the code from step 3 with this one:
document.cookie="VISITOR_INFO1_LIVE=; path=/; domain=.youtube.com";window.location.reload();{ Thanks, Rubén and Ramona Brown. }