The guideline requires that the page is valid.

Make sure that the page contains valid code. Use the Markup Validator, the XML Validator or a CSS validator to check if there are problems and to find viable solutions.

Make sure the page contains absolutely valid code.

By using appropriate coding standards, it is possible to develop documents that are guaranteed to be potentially (and eventually) compatible with all types of Internet browsers and agents.

In contrast, coding for a specific browser type and version, May lead to documents that are unreadable as new versions of that browser are developed, and as new browsers are used. Consider that in addition to mainstream browsers, website visitors may also use speaking browsers (e.g. Home Page Reader), text only browsers (e.g. lynx), or mini browsers on PDAs and cell phones. In addition to human visitors, the site may be visited by crawlers of search engines and of more specialized information gathering agents. These will also benefit from a document that is compliant with known standards.

Using syntax validators will help to determine if there are mistakes in the documents; mistakes that were inserted inadvertently into the document that could lead to unexpected, and sometimes undetected, accessibility issues.