The Tor Browser / file revision

 <window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"

         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 vbox = document.getElementsByTagName("vbox")[0];

       var span = document.createElementNS(HTML_NS, "html:span");

       vbox.insertBefore(span, vbox.firstChild); // causes block wrapping

       setTimeout(finish, 0);

     }

     function finish() {

       document.documentElement.removeAttribute("class");

     }

     function load(event) {

       setTimeout(run, 0);

     }

     window.addEventListener("load", load, false);

   ]]></script>

   <vbox flex="1">

     <box flex="1" />

     <label value="hello world" />

   </vbox>

 </window>