gecko-dev/layout/reftests/box-shadow/boxshadow-dynamic.xul

<?xml version="1.0"?>
<?xml-stylesheet type="text/css" href="data:text/css,

hbox {
  margin: 50px;
  background: yellow;
}

hbox[x] {
  box-shadow: 1px 2px 2px black, 0 0 7px white, 0 0 14px rgb(50, 170, 255), 0 0 21px rgb(50, 170, 255);
  border-radius: 10px;
}

"?>
<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
        class="reftest-wait">
  
  <script>
  <![CDATA[
  
  document.documentElement.addEventListener("MozReftestInvalidate", 
    doTest, false);

  function doTest() {
    document.getElementById("hbox1").setAttribute("x",1);
    document.documentElement.removeAttribute("class"); 
  }
  
  ]]>
  </script>

  <hbox id="hbox1" flex="1"/>
</window>