This site takes a completely different approach. It is examining your page for accessibility by people with disabilities (such as total or partial blindness), as well as browser compatibility. 'Bobby', in case you don't know, is a term for an English policeman and this site uses a bobby's helmet as an icon superimposed on a reproduction of your page everywhere that it finds something to complain about. Clicking on this helmet icon takes you to a note further down the page that explains what is wrong, and makes suggestions as to how you can fix it.
The site focuses on making sure that you have ALT tags for all you images, even encouraging you to use extended ALT tags and textual explanations of animations and charts for people who are listening to rather than looking at Web pages.
Using the `advanced' page, you can specify which version of HTML or which browser and version you want to check against including WebTV. Unfortunately, it hasn't yet added support for Mac Explorer 5 and Netscape 6.
Although it is not the most cutting edge HTML validator, it is certainly the most user-friendly syntax checker I've encountered and you can download a free offline version of the program written in Java to check your pages before uploading them to a server.
When your pages validate with Bobby, you can append a `Bobby Approved' badge to them.