<html>
<head>
<script type="text/javascript">

function boom()
{
  function x()
  {
    document.removeEventListener("DOMAttrModified", x, false);
    document.execCommand("backcolor", false, "green");
  }

  document.getElementById("td").focus();

  document.addEventListener("DOMAttrModified", x, false);
  try { 
    document.execCommand("subscript", false, null); 
  } catch(e) { 
  }
  document.removeEventListener("DOMAttrModified", x, false);
}

</script>
</head>

<body contenteditable="true" onload="setTimeout(boom, 30);">
<table><tbody contenteditable="false"><tr><td contenteditable="true" id="td"></td></tr></tbody></table>
</body>

</html>