Alright this is simple, and probably common sense to some, but I thought it was worth a quick entry for anyone scratching their heads.
YouTube spits out this bit if you want to share or embed a video:
<iframe width="420" height="315" src="http://www.youtube.com/embed/RqbWdo536as?rel=0" frameborder="0" allowfullscreen></iframe>
Both the frameborder and allowfullscreen attributes are what make this code invalid according to the W3C.
<iframe src="http://www.youtube.com/embed/RqbWdo536as?rel=0" style="border:none;width:420px;height:315px;"></iframe>
Although I don’t recommend using inline styles (I recommend the style be placed in your stylesheet), by removing the two invalid attributes and using a style in its place I now have W3C valid code… AND I have better control over the iframe as well. The allowfullscreen attribute does nothing since the full-screen button is in the play bar. Notice I also moved the size of the iframe into the style as well. With many videos on a page and a single stylesheet this method will end up saving you a lot of work anyhow.
Why YouTube doesn’t do this by default though goes beyond me.