HQ on youtube

YouTube already provides an option to view higher quality versions for some videos and you can also append "&fmt=18" to the URLs for the other videos. If you add "&fmt=22", some YouTube videos are now available at an even higher quality, which makes the full screen view a very useful option. Compare:

http://www.youtube.com/watch?v=zlfKdbWwruY (standard format - mono, resolution: 320x180)
http://www.youtube.com/watch?v=zlfKdbWwruY&fmt=18 (stereo, resolution: 480x270)
http://www.youtube.com/watch?v=zlfKdbWwruY&fmt=22 (new format - stereo, resolution: 1280x720)

To embed a higher quality version of a video, replace "http://www.youtube.com/v/VIDEO_ID" with "http://www.youtube.com/v/VIDEO_ID&ap=%2526fmt%3D18" or "http://www.youtube.com/v/VIDEO_ID&ap=%2526fmt%3D22" in YouTube's embeddable code.


Source