According to the ASP.NET Docs on XHTML Conformance if you try to validate an ASP.NET page with the W3C XHTML validation service it might not report it as valid XHTML simply because the W3C validation service does not report itself as a browser that ASP.NET recognises.

You can get around this problem by creating a browser definition in a .browser file that you put into your App_Browsers folder of your ASP.NET website.

Yoinked directly from the docs:

<browsers>
  <browser id="W3C_Validator" parentID="default">
    <identification>
        <userAgent match="^W3C_Validator" />
    </identification>
    <capabilities>
      <capability name="browser"              value="W3C Validator" />
      <capability name="ecmaScriptVersion"    value="1.2" />
      <capability name="javascript"           value="true" />
      <capability name="supportsCss"          value="true" />
      <capability name="tables"               value="true" />
      <capability name="tagWriter" 
         value="System.Web.UI.HtmlTextWriter" />
      <capability name="w3cdomversion"        value="1.0" />
    </capabilities>
  </browser>
</browsers>

This might just be handy to know at some point, although most good browser based web dev tool suites offer the ability to validate XHTML, and I believe they send the content to validate from rendered markup in your browser.