some things that w3c validator could warn/complain about (mostly XHTML interoperability issues)

There are a few things that the validator could warn about:

- empty class="" attribute does not seem to be allowed, but is not 
reported by the validator
- HTML-comments inside of CSS and script tags are legal XHTML, but will 
cause content to be ignored by XHTML-conformant browsers
- tags with non-empty content model in HTML4 should be closed explicitly 
in XHTML to ensure interoperability; the validator could warn in such 
situations when validating XHTML
  (e.g. <a id="foo" name="foo"></a> and not <a id="foo" name="foo" />)
- named HTML entities like &copy; are legal in XHTML when specifiying a 
DTD, but a non-validating browser is free to ignore the DTD and thus not 
know them; thus when using named entities except &gt; / &lt; / &quote; / 
&amp; in XHTML, the validator should warn

Best regards and thanks for the validator service
Christian