The World Wide Web Consortium (W3C) creates web standards.
Site validation service
One of the most used services is the validation service to control code of websites.
The validator controls markup validity from documents on the Internet in HTML, XHTML, SMIL, MathML, etc. There are other tools to control specific content such as RSS/Atom feeds or CSS stylesheets or to find broken links, there are other validators and tools.

Not all programmers always agree with the way W3C does it but the validation tool gives a good idea about code quality, and it is correct that it can happen that an error is given by the validation tool that is not really a coding error or mistake. Having the standards set by W3C gives programmers good guidelines.

More info on:

www.w3.org

Validate a site on:

http://validator.w3.org/