Edit in JSFiddle

var showDD = function () {
    var b = document.getElementById('b');
    b.innerHTML = 'document.domain = ' + document.domain;
}
var changeDD = function () {
    document.domain = 'jshell.net';
    showDD();   
}
showDD();
$('#btn').bind('click', changeDD);

<div id=a><div id=b></div><button id='btn'>Update document.domain</button></div>
<iframe src="http://fiddle.jshell.net/mBBaQ/4/show" width="300" height="130" seamless scrolling=no></iframe>

#a {
    display: block;
    background: lightgrey;
    width: 300px;
    height: 130px;
}