Question:

How can I display the YouTube video in hd in HTML5 LightBox?

Answer:

You can add a parameter vq=hd720 to the YouTube URL, for example:

<a href="http://www.youtube.com/embed/YE7VzlLtp-4?vq=hd720" class="html5lightbox" title="Big Buck Bunny Copyright Blender Foundation">YouTube Video</a>

The demo is as following:

Show YouTube Video in HD in LightBox

You can also define the video quality as 1080p with parameter vq=hd1080.

If the specified video quality is not available, YouTube will automatically downgrade to the lower quality.