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://home.snafu.de/tusk/mod_tidy/