The XHTML validator in the editor control is not working.
As per attached screenshot, when you click it and opens the window it simply states the following.
"Sorry! This document cannot be checked."
"Sorry, this type of URL scheme (undefined) is not supported by this service. Please check that you entered the URL correctly"
Can someone tell me why this is happening? It's also broken on the editor demos on your site. Maybe something on the W3C validator has changed making it no longer compatible/viable?