You know, how to loop video & music in desktop media player, but if you want to play a YouTube embedded video in an infinite loop, you can automate the process and play the video in loop in two ways:

• use Loopy for YouTube, a Greasemonkey script that adds a small link below the video to start the loop. The script requires Greasemonkey , a Firefox extension, and it's efficient: it doesn't reload the page to download the video again.
• There's another way to play a YouTube video in a loop - use the loop parameter of the embeddable player (here's the complete list of parameters):

  http://www.youtube.com/v/VIDEOID&loop=1 (replace VIDEOID with the video's ID)

  If you add the autoplay parameter, the video will start to play automatically:

  http://www.youtube.com/v/VIDEOID&loop=1&autoplay=1

Source:→ Google System