title: JSF xmlns URI not registered in IntelliJ IDEA
body: <p>Here is the start of my JSF file:</p>
  <pre><code>&lt;?xml version='1.0' encoding='UTF-8' ?&gt;
  &lt;!DOCTYPE html PUBLIC &quot;-//W3C//DTD XHTML 1.0 Transitional//EN&quot; &quot;http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd&quot;&gt;
  &lt;html xmlns=&quot;http://www.w3.org/1999/xhtml&quot;
        xmlns:h=&quot;http://java.sun.com/jsf/html&quot;
        xmlns:ace=&quot;http://www.icefaces.org/icefaces/components&quot;
        xmlns:c=&quot;http://java.sun.com/jsp/jstl/core&quot;
        xmlns:f=&quot;http://java.sun.com/jsf/core&quot;&gt;
  &lt;h:head&gt;
  &lt;/h:head&gt;
  &lt;h:body&gt;
  </code></pre>
  <p>However, <code>http://www.icefaces.org/icefaces/components</code> and <code>http://java.sun.com/jsp/jstl/core</code> are highlighted in red with error <code>URI is not registered</code>. This causes many other subsequent tags to appear in red with errors <code>element 'x' is not allowed here</code>. Here is an image showing what I mean:</p>
  <p><img src="https://i.stack.imgur.com/2pc9W.png" alt="xmlns and tags appear in red" /></p>
  <h2>Attempted solutions:</h2>
  <p>When searching for a solution, it appeared some others with the same problem solved it by doing &quot;Fetch External Resource&quot;. However, this didn't work for me. For the icefaces xmlns <code>xmlns:ace=&quot;http://www.icefaces.org/icefaces/components&quot;</code> I get the error: <code>Error while fetching http://www.icefaces.org/icefaces/components</code>. And for the jstl xmlns <code>xmlns:c=&quot;http://java.sun.com/jsp/jstl/core&quot;</code> I get the error: <code>No XML at the location: http://java.sun.com/jsp/jstl/core</code>.</p>
  <p>I also found some people solved it by deleting the IntelliJ IDEA metadata file <code>workspace.xml</code> in the <code>.idea</code> directory. This does seem to work temporarily... but then it seems the problem comes back after a while. I also have to reconfigure several project settings which I would like to avoid.</p>
  <p>Any help would be appreciated.. I'm not really familiar with the xmlns stuff and still a beginner to web app development. Thank you!</p>
  <h2>Update:</h2>
  <p>As per Mark Vedder's answer, I manually added external resources to my project and the problem has not resurfaced thus far.</p>