Not XHTML! How Do I make it so?

<input type="radio" name="sitesearch" value="" checked />

The checked component makes for a non-XHTML compliant web page according to
W3C's Validator.  I've done quite a bit of research and have found nothing
to make sense of it.  Putting it in quotes does no good either.  You can see
the page of the code at http://www.shortfamilyonline.com/links/ 

I appreciate any help in this matter.  Thank you.