title: HTML: What is `xmlns:fb="http://www.facebook.com/2008/fbml"`?
body: <p>What is <code>xmlns:fb="http://www.facebook.com/2008/fbml"</code>?</p>
  
  <p>I've been seeing it in a lot of <code>&lt;html&gt;</code> tags recently. What does it do?</p>