title: Adding xmlns protocols to xml namespace in HTML 5
body: <p>So as an example, we have this head taken from <a href="http://developers.facebook.com/docs/opengraphprotocol/" rel="nofollow noreferrer">Facebook's open graph protocol</a></p>
  
  <pre><code>&lt;html xmlns="http://www.w3.org/1999/xhtml"
        xmlns:og="http://ogp.me/ns#"
        xmlns:fb="http://www.facebook.com/2008/fbml"&gt;
  </code></pre>
  
  <p>which would be fine with an xhtml doc type.
  But what about HTML 5?
  I have searched for answers, including previous stackoverflow questions and came up short.
  I know that the xmlns attritbute is allowed in HTML5 if its value is exactly "<a href="http://www.w3.org/1999/xhtml" rel="nofollow noreferrer">http://www.w3.org/1999/xhtml</a>". But are attributes with prefixes in the format of xmlns:ex allowed under HTML5? And if they are, is the xmlns attribute necessary when using them? I know that the attribute is not necessary if you have no other attributes that use it.
  Here is <a href="https://stackoverflow.com/questions/6372362/html-5-adding-xml-namespaces">practically my same question</a> but the answer doesn't seem to answer the question.
  And <a href="https://stackoverflow.com/questions/1081571/facebook-connect-and-html5-xmlns-available/1081704#1081704">another question</a> asks about HTML5, facebook, and xmlns.
  There is only one comment that mentions:</p>
  
  <blockquote>
    <p>Only the xmlns="w3.org/1999/xhtml"; attribute is allowed on . >xmlns:fb="facebook.com/2008/fbml"; is invalid per HTML5. – hsivonen Jul 6 '09 at 12:05</p>
  </blockquote>
  
  <p>So is that true? You can't use Facebook Open Graph with HTML5?!</p>