<script>
        window.onload = function(){
                var aDoc = document.implementation.createDocument("", "", null);
                var aElem = document.createElement('DIV');
                aElem.innerHTML = "<table id='ptable'><tr><td>this is a table</td></tr></table>";
                aDoc.appendChild(aElem);
                var ret = aDoc.evaluate("//table[@id='ptable']", aElem, null,XPathResult.FIRST_ORDERED_NODE_TYPE, null);
                alert("navigator.userAgent : " + navigator.userAgent + "\r\n\r\n" + "ret.singleNodeValue : " + ret.singleNodeValue);
        }
</script>

 alert(aElem.innerHTML);

var nsResolver = {
  lookupNamespaceURI:function (prefix) {
    if (isFF36up && prefix == "ns") {
      return "http://www.w3.org/1999/xhtml";
    }
    else {
      return "";
    }
  }
};

var ret = aDoc.evaluate("//ns:table[@id='ptable']", aElem, nsResolver,XPathResult.FIRST_ORDERED_NODE_TYPE, null);