Youtube's embeded players ignore the &fmt=18 command added onto the embeded videos URL and play the video in standard quality.

To get the embeded video to play in High Quality you add &ap=%2526fmt%3D18 onto the URL instead.

So, for example

<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/19qemq6mWuY&hl=en&fs=1"></param><param name="allowFullScreen" value="true"></param><embed rel="lightbox" src="http://www.youtube.com/v/19qemq6mWuY&hl=en&fs=1" type="application/x-shockwave-flash" allowfullscreen="true" width="425" height="344"></embed></object>

becomes

<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/19qemq6mWuY&ap=%2526fmt%3D18&hl=en&fs=1"></param><param name="allowFullScreen" value="true"></param><embed rel="lightbox" src="http://www.youtube.com/v/19qemq6mWuY&ap=%2526fmt%3D18&hl=en&fs=1" type="application/x-shockwave-flash" allowfullscreen="true" width="425" height="344"></embed></object>