This page shows some limitations of DTDs, which the W3C Validator almost exclusively uses for validity checking (beyond its initial well-formedness check). Many rules in the XHTML specification cannot be expressed in a DTD, so the Validator misses a lot of errors.

Validate me!

On elements other than form controls, name and id are supposed to be synonymous. But here we're using them like name has nothing to do with anything.

A link in a link in a link

A div inside a span? NO WAI!

Nested forms!

Check out the source to see the full whackiness. We have incorrectly nested elements and horribly wrong attribute values. Just because the validator says it's okay doesn't mean it is.

Also check out the HTML version.