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        xmlns:html="http://www.w3.org/1999/xhtml"
        class="reftest-wait">
  <script type="text/javascript"><![CDATA[
    function run() {
      const XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
      const HTML_NS = "http://www.w3.org/1999/xhtml";
      var window = document.getElementsByTagName("window")[0];
      var span = document.createElementNS(HTML_NS, "html:span");
      window.appendChild(span); // causes block wrapping
      setTimeout(finish, 0);
    }
    function finish() {
      document.documentElement.removeAttribute("class");
    }
    function load(event) {
      setTimeout(run, 0);
    }
    window.addEventListener("load", load, false);
  ]]></script>
    <box flex="1" />
    <label value="hello world" />
</window>