title: Element <element> is not a known element warning in Visual Studio when using User controls
body: <p>I have a Visual Studio 2008 project that is showing the following warning when using User Controls, and I haven’t been able to find a solution anywhere.</p>
  
  <blockquote>
    <p>Element <code>&lt;element&gt;</code>is not a known element </p>
  </blockquote>
  
  <p>How can I fix this?</p>