CheckSite is a tool that can be used to find potential problems in HTML. It uses the W3C Validator to validate the page, and then performs a number of additional checks. These checks are "tribal knowledge", things that are not part of an official standard but are known to be best practice.
Checks are divided into a number of categories: browser compatibility (does the page use constructs that are known to cause problems in some browsers?), SEO (does the page adhere to SEO guidelines?), user experience (does the page have a favicon and sitemap?), and internationaliation (does the page use UTF-8 encoding?).
CheckSite doesn't simply validate HTML. Instead, it checks against a number of known compatibility problems with browsers, to ensure that a site will display properly even when someone is using an older or obscure browser.