title: Whitelist element with class of, using htmlpurifier
body: <p>I want to only allow the span element only when it has a certain class in htmlpurifier </p>
  
  <p>does anyone know how to do this, right now I have</p>
  
  <pre><code>  $config-&gt;set('HTML.Allowed','a[href],p,ol,li,ul,img[src],blockquote,em,span[class]');
    $config-&gt;set('Attr.AllowedClasses',"allowed");
  </code></pre>
  
  <p>but that allows all spans and only allows class allowed  I like that it only allows the "allowed" class but I only want it to allow span when the value of its class is "allowed"</p>
  
  <p>thanks</p>