With the quantity of mobile devices that surfing in the net, and the recent news about not flash support on the new devices, the integration of html5 is needed for compatibility.
So, if you want to force HTML5 and HD for an embedded video, you can use this code:
<iframe src="http://www.youtube.com/embed/pmPNz8JJCm8?html5=1&vq=hd720"></iframe>
Below you can see an example:
