function alertContents() {
      if (http_request.readyState == 4) {
         if (http_request.status == 200) {
            //alert(http_request.responseText);
            result = http_request.responseText;
            document.getElementById('myspan').innerHTML = result;            
         } else {
            alert('There was a problem with the request.');
         }
      }
   }

<script>alert()</script>

var node = document.getElementById('myspan');
var text = document.createTextNode(result);
node.innerHTML = '';
node.appendChild(text);