Rated 5 out of 5 stars
If PsilentKnight had bothered to read the docs, he's know he was wrong.
If you install the desktop tool, all validation is done locally (try it, it works even if you're offline), and because the full page source (not just the URL) is sent to the local app, no authentication is necessary.
The online service is obviously a different matter, but it would be hard to generate browser previews without page source, and it doesn't necessarily represent a risk as it never needs to allow external authentication anyway.
UI is not as nice as the defunct HTML Validator plugin, but it works really well, definitely a good recommendation.