title: xmllint validation error "no DTD found" while using XSD
body: <p>I'm trying to use xmllint to check my work while developing a basic XSD i.e. XML Schema schema. However it's giving me an error</p>
  
  <blockquote>
    <p>Validation failed: no DTD found.</p>
  </blockquote>
  
  <p>What am I doing wrong?</p>
  
  <p>My xmllint command:</p>
  
  <pre><code>xmllint --noout --valid --schema simple.xsd lucas-basic.xml
  lucas-basic.xml:5: validity error : Validation failed: no DTD found !
          &gt;
          ^
  lucas-basic.xml validates
  </code></pre>
  
  <p>Test XSD file:</p>
  
  <pre><code>&lt;?xml version = "1.0" encoding = "UTF-8"?&gt;
  &lt;!--Generated by XML Authority. Conforms to w3c http://www.w3.org/2001/XMLSchema--&gt;
  &lt;xsd:schema xmlns:xsd = "http://www.w3.org/2001/XMLSchema"&gt;
          &lt;xsd:element name = "vehicles"&gt;
                  &lt;xsd:complexType&gt;
                          &lt;xsd:sequence&gt;
                                  &lt;xsd:element name = "nickname" type = "xsd:string" maxOccurs = "unbounded"/&gt;
                          &lt;/xsd:sequence&gt;
                  &lt;/xsd:complexType&gt;
          &lt;/xsd:element&gt;
  &lt;/xsd:schema&gt;
  </code></pre>
  
  <p>Test XML file:</p>
  
  <pre><code>&lt;?xml version="1.0"?&gt;
  &lt;vehicles
                  xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance"
                  xsi:noNamespaceSchemaLocation = "http://lucas.ucs.ed.ac.uk/xml-schema/xmlns/simple.xsd"
          &gt;
          &lt;nickname&gt;Bog Hopper&lt;/nickname&gt;
          &lt;nickname&gt;Wee Beastie&lt;/nickname&gt;
          &lt;nickname&gt;Count Zero&lt;/nickname&gt;
  &lt;/vehicles&gt;
  </code></pre>
  
  <p>The URL at xsi:noNamespaceSchemaLocation does return the above XSD. Also, I downloaded the xsd file and put it into the current directory as an extra measure, but this didn't seem to have changed anything.</p>