It will also check your Cascading Style Sheets for correctness, but from another starting page . You simply enter the URL of the page you want to check and a few seconds later, it comes back with a full report. The report lists each line that it considers to have an error. Although your page may work perfectly well in your browsers, these errors refer to their compliance with W3C HTML standards, not browser compatibility, and as we know, W3C compatible browsers are very rare beasts indeed. The use of NAME for forms and images.
Most browsers can, and do use the NAME attributes.
Missing ALT tags for images. Unfortunately, the W3C Validator just gives an absolute PASS or FAIL for the whole document. There are no degrees of correctness even though none of these errors cause problems with any browser that I've tested them in so far. It would be nice if there was more explanation of how to fix such errors rather than just point at 'the book' and say 'go find it'. This site takes a completely different approach.