To link to a YouTube video so it plays in HD by default, use the following URL:
https://www.youtube.com/v/VIDEOID?version=3&vq=hd1080
Change VIDEOID to the YouTube video ID that you want to link to. When someone follows the link, it will display the highest-resolution available (up to 1080p) in full-screen mode. Unfortunately, vq=hd1080 does not work on the normal YouTube site (with comments and related videos).
