Everyone can make a website nowadays. Or at least, that's what we'd like to think. In reality, even though Wordpress and Content Management Systems have made it a lot easier to put up a brochure website or blog, the large majority of websites is still custom-made.
For most programming languages, tools have been developed to automate testing, find potential problems, and measure code quality. Examples are JUnit and FindBugs. Strangely enough, few of this tools exist for HTML. The W3C Validator checks if a page is well-formed, but it does not check if the page has a favicon, or if images have empty alt texts.
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.