There are several ways how to get this working. You can either use jquery like this:
$('#mydiv').bind("DOMSubtreeModified",function(){
// your code goes here
alert('changed');
});
Note that this is not supported in IE8( and below).
Or you can run loop and continuously fire the desired code:
var interval = setInterval(function() {
// your code goes here
}, 1000);
Fiddle: http://jsfiddle.net/8RF5r/5/