Thank you, Lynne, for the well thought out article. I have been trying to figure this out for 2 days now. You made it easy, young lady.
For anyone interested, here is all you need to make a W3C button that will check the markup validation for whatever page of the site you are currently on.
PHP Code:
$template_hook['navtab_end'] .= '<li><a class="navtab" href="http://validator.w3.org/check?uri=referer" target="_blank">W3C</a></li>';