title: Resolving "Validation (): Element ‘xxxx’ is not supported" warning in Visual Studio 2005/2008
body: <p>Over the last couple of days I started getting the following validation warning (green squiggly line at design time) in a Visual Studio 2008 ASP.NET WebForms project:</p>
  <blockquote>
  <p>Validation (): Element ‘xxxx’ is not supported.</p>
  </blockquote>
  <p>Where 'xxxx' was a standard ASP.NET server control (asp:panel, asp:container, asp:textbox)</p>
  <p>The application would still compile and run but the warnings were an annoyance.</p>
  <p>See my answer for the resolution I found.</p>