function processReqChange() {
    // only if req shows "loaded"
    if (http.readyState == 4) {
        if (http.status == 200) {
       document.submitLink.categories.disabled = false;

            alert("We're good to go...");
       var XMLResponse = http.responseXML;
      
       var product_list = XMLResponse.getElementsByTagName("list");
       var product_id   = XMLResponse.getElementsByTagName("category");

       alert("SIZE: " + product_list.length + "x" + product_id.length); // returns: SIZE: 1 x 3

       var select = document.submitLink.categories;
       //clearTopicList(select);

       for (var i = 0; i < product_id.length; i++) {
      alert("i = " + i + " " + product_id[i].getAttribute('id') + " " + product_id[i].getAttributeNode('name'));
       }

       alert('DONE.');
      
        } else {
            alert("There was a problem retrieving the XML data:\n" +
               http.statusText);
        }
    }
}

<list>
  <category id="1">
     <name>History</name>
  </category>
  <category id="2">
     <name>Art</name>
  </category>
  <category id="3">
     <name>CSC</name>
  </category>
</list>

product_id[i].getAttribute('id').childNodes.item(0)