mod_tidy validates the HTML output of your apache2 webserver. So you don't need 
a separate application to check your HTML.
It works as a filter that hooks up to HTML output. mod_tidy feeds the HTML 
output to TidyLib which validates the HTML output. If TidyLib finds 
an error the client receives a HTML page with a list of all found errors. If 
TidyLib doesn't complain you will get your HTML data as without mod_tidy.

WWW: http://mod-tidy.sourceforge.net/