In order to automatically play YouTube videos in HD you are going to need an account with YouTube. Then, sign in to your account and click the Account link located in the upper right hand corner of the screen.

From there, click on Playback Setup located along the left hand side of the screen and choose the “I have a fast connection. Always play higher-quality video when it’s available.” and that is all that there is to it. Assuming your connection is fast enough and the video is available in HD format, YouTube will now always load the video in HD giving you a much better picture as well as better audio.