You want to embed a YouTube video in your XHTML 1.0 document, but the code that YouTube gives you does not validate. Instead, use something like:
<object type="application/x-shockwave-flash" width="480" height="385"
data="http://www.youtube.com/v/_ZG8HBuDjgc"><param name="movie"
value="http://www.youtube.com/v/_ZG8HBuDjgc" /><param name="wmode"
value="transparent" /></object>
