The Markup Validation Service by the W3C allows to check HTML documents for conformance to their HTML and XHTML standards. Although HTMl files will render regardless of errors in major browsers, they often contain many structure and/or form errors. The Markup Validation Service is not only a good way to make sure you're conforming to standards, but is a quick method to check for errors in your code. It is also a sneaky way for the W3C to enforce their wanted movement of HTML and webpage standards.
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