i think that a plugin doing a w3 html validation would be a good and useful thing. and i don't think that it would be uch of a trouble to build one, since it would use the online validator, or a library, or something like this. don't really know how to make it myself thou :)
The TextFX plugin already does this and submit CSS. It will submit as much of the file as you mark or the entire file if you don't mark anything. The required files for this function may not have been distributed with your Sourceforge download, and if not, get the full package from my web page. If you use a different validator than I chose or the validator method changes in the future, the files that help provide this function should be easy to modify.
Someone who shall remain nameless has been guilty of not using their own validator tool.