WeirdAl

<?xml version="1.0"?>
<?xml-stylesheet type="text/css" href="chrome://global/skin/"?>
<window
  xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
  <vbox>
    <label value="Input:"/>
    <hbox>
      <textbox multiline="true"
               rows="15"
               cols="80"
               id="xml-source"
              />
    </hbox>
    <label value="Parser results:"/>
    <hbox>
      <textbox multiline="true"
               rows="5"
               cols="80"
               readonly="true"
               id="results"
              />
    </hbox>
  </vbox>

  <script type="application/javascript">
const saxReader = Components.classes["@mozilla.org/saxparser/xmlreader;1"]
                            .createInstance(Components.interfaces.nsISAXXMLReader);

const xmlErrorHandler = {
  reset: function reset() {
  },

  error: function error(locator, message) {
  },

  fatalError: function fatalError(locator, message) {
  },

  warning: function warning(locator, message) {}
};

saxReader.errorHandler = xmlErrorHandler;

const results = document.getElementById("results");

const xmlSource = document.getElementById("xml-source");
xmlSource.addEventListener("input", function parse(evt) {
  try {
    saxReader.errorHandler.reset();
    results.value = "Passed";
  }
  catch (e) {
    results.value = e;
  }
}, true);
  </script>
</window>