I found that it's an intractable pig of a thing.
It works, but there are better options.
https://addons.mozilla.org/en-US/firefox/addon/249That one rocks. The error readouts are far more user-friendly that the W3 gizmo and it runs the same algorithm so it checks to the same standard. Added bonus is that it will validate any page you have access to, which means you can work on a local host and still validate your pages. I never use W3 any more.
ETA: Oh and it's much faster too.
Devious, treacherous, murderous, neanderthal, sub-human of the West.