title: Define a DTD ATTLIST to allow a new attribute to be without a value?
body: <p>I have the following:</p>
  
  <pre><code>&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
     "http://www.w3.org/TR/html4/loose.dtd"
  
      [
          &lt;!ATTLIST a extra-attr CDATA #IMPLIED&gt;
      ]&gt;
  
  &lt;html&gt;
    &lt;head&gt;
      &lt;title&gt;Title&lt;/title&gt;
    &lt;/head&gt;
    &lt;body&gt;
      &lt;a extra-attr&gt;hola&lt;/a&gt;
    &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>What I need is:</p>
  
  <ol>
  <li>The attribute value  present: <code>&lt;a extra-attr="value"&gt;&lt;/a&gt;</code> should be valid</li>
  <li>The attribute value not present: <code>&lt;a extra-attr&gt;&lt;/a&gt;</code> <strong>valid as well</strong></li>
  </ol>