The W3C Markup Validation Service is a web gateway to a well known SGML parser called SP. SP will take the HTMLin your website and compare it to a set of objective syntax rules called a "DTD", a Document Type Definition. This way you can be sure the HTML in your website is really valid and not just that it conforms to some random programmer's idea of "nice" HTML. Note that valid HTML does not guarantee that your web pages will work OK in all browsers. Most of them are severely broken and you may need to find alternate ways of achieving your goal.

When you send an URL to the W3C Markup Validation Service, it will fetch that URL and feed it to the SGML parser. If you upload a file it'll get fed directly into the SGML parser. We then take the output from the SGML parser and format it nicely as HTML and send it back to your web browser. The W3C Markup Validation Service isn't generating any of the error messages; they are all generated by the underlying SGML Parser which is checking your HTML against the actual standard for the version of HTML you are using.

Northern Web Technology will validate every page of your website through the W3C Markup Validation Service and award your website with the W3C approval for HTML 4.01 Standards.