Why won't my xhtml validate?

I am re-writing my site (www.cirt.f2o.org) to meet xhtml 1.1 standards.
Then, I registered my domain name (www.cirt.us), and because my webhost
doesn't support DNS, I have a "stealth redirect" to my real url. Because
the "stealth redirect" uses a frame to hide the true URL of my site, I had
to add a target="_top" attribute to all of my off-site links.
unfortunately, the w3c validator flags this as an error, even though
"target="_top" is listed in the "target" module of the xhtml 1.1 specs...
so why doesn't the validator recognise this? If I remove the target
attribute, the site validates.

Help?