title: what is attribute.specified for a DOM element's attributes?
body: <p>I just stumbled across something like this...</p>
  
  <pre><code>function(element) {
    ...
    var attributes = element.attributes;
    for (var index = 0, length = attributes.length; index &lt; length; index++) {
      var attribute = attributes[index];
  
      // what is ".specified"?
      if (attribute.specified) {
        ...
      }
    }
  }
  </code></pre>
  
  <p>I'm looking at the W3C specs for a DOM Element, the Element interface, and I don't see <code>specified</code> anywhere. </p>
  
  <p><a href="http://www.w3.org/TR/DOM-Level-3-Core/core.html#ID-745549614" rel="noreferrer">http://www.w3.org/TR/DOM-Level-3-Core/core.html#ID-745549614</a></p>
  
  <p>What does <code>attribute.specified</code> mean? What does it represent? Where is it defined in the specs?</p>