<!DOCTYPE html>
<html>
<head>
<style type='text/css'>
div {
height: 20px;
background-color: red;
}
</style>
</head>
<body>
<script type='text/javascript'>
var f = Element.prototype.appendChild;
Element.prototype.appendChild = function(){
f.apply(this, arguments);arguments[0].innerHTML="!Intercepted!";
};
document.body.appendChild(document.createElement("div"));
</script>
</body>
</html>