<!DOCTYPE html>
<body>
<iframe id="testFrame"></iframe>
<script>
document.getElementById('testFrame').contentDocument.body.innerHTML = "<div id='testElement' style='display: none'>TEST</div>"

setInterval(function() {
  // Change the style of a display:none element.
  var testFrame = document.getElementById("testFrame");
  var testElement = testFrame.contentDocument.getElementById("testElement");
  testElement.style["left"] = "" + Math.floor((Math.random() * 10) + 1) + "px";
}, 0);
</script>
</body>