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                <h1>Controller Window</h1>
  This document is on the domain:
  <button id="send">Send Message</button>

<iframe id="receiver" src="" width="500" height="200">
  <p>Your browser does not support iframes.</p>


                body {
  font-family: Helvetica, Arial, sans-serif;
  background: #333;
  color: #FFF;
  border-top: 5px solid #69c773;
  padding: 0 1em;
  margin: 0;

iframe {
  background: #FFF;
  border: 3px solid #000;


                window.onload = function() {
	// Get the window displayed in the iframe.
	var receiver = document.getElementById('receiver').contentWindow;
	// Get a reference to the 'Send Message' button.
	var btn = document.getElementById('send');

	// A function to handle sending messages.
	function sendMessage(e) {
		// Prevent any default browser behaviour.

		// Send a message with the text 'Hello Treehouse!' to the new window.
		receiver.postMessage('Hello Treehouse!', '');

	// Add an event listener that will execute the sendMessage() function
	// when the send button is clicked.
	btn.addEventListener('click', sendMessage);