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 is not just a HTML validator. For example, it contains specific checks for features that don't work in obscure old browsers. Also, instead of just giving a number of problems in the page CheckSite gives you a summary of the severity of these problems.
This software is designed for people who work with websites, either in web development, web design, or SEO.