title: DOCTYPE declaration is missing the system identifier
body: <p>I'm receiving the following response from a web server.</p>
  
  <pre><code>&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
  "http://www.w3.org/TR/html4/strict.dtd"&gt; &lt;HTML&gt;&lt;HEAD&gt;&lt;TITLE&gt;Service Unavailable&lt;/TITLE&gt; &lt;META HTTP-EQUIV="Content-Type" Content="text/html; charset=us-ascii"&gt;&lt;/HEAD&gt; &lt;BODY&gt;&lt;h2&gt;Service Unavailable&lt;/h2&gt; &lt;hr&gt;&lt;p&gt;HTTP Error 503. The service is unavailable.&lt;/p&gt; &lt;/BODY&gt;&lt;/HTML&gt;
  </code></pre>
  
  <p>I'm a bit confused as to whether or not this html is valid.</p>
  
  <p>When I try to parse the document in my code I get a <code>White spaces are required between publicId and systemId.</code> which indicates that the DOCTYPE declaration is missing the system identifier. But then when I go to the W3C validator it seems to pass validation.</p>
  
  <p>Is it valid HTML?</p>