If you have to link to a 720p HD video on YouTube from your web pages or share those links in an email, here’s a simple hack – just append &hd=1 to the YouTube video URL and it will directly play the high-res widescreen version of that video in the browser.
For instance, the URL youtube.com/watch?v=ZOmLnGrq-bA links to the standard (grainy) version of the YouTube video while youtube.com/watch?v=ZOmLnGrq-bA&hd=1 will prompt YouTube to show the high quality HD version of that video.
This is more easy than all the previous parameters (like &fmt=22 or &fmt=18) as they weren’t supported officially and therefore your video links could break in the future. [via]
Related: Search HD Videos on YouTube