Bookmarklet

How about offering people a <a 
href="javascript:void(window.open('http://validator.w3.org/check?uri='+escape(location),'W3 
Validation','location=no'))">bookmarklet</a>?

With it you can validate pages without buttons at a click. I find it 
best deployed on your bookmarks toolbar.

Link source as follows
javascript:void(window.open('http://validator.w3.org/check?uri='+escape(location),'W3 
Validation','location=no'))

Matt