In my previous blog entry, I showed how to configure ASP.NET to output XHTML to the browsers. However, when the W3C Validator checks the page, ASP.NET does not know the W3C user agent and hence sends out non compliant code. The trick is in the w3cvalidator.browser file available on idunno.org. Just drop the file in your App_Browsers folder and ASP.NET will send out XHTML compliant code to the Validator.