gareth/index.html

## index.html
<!DOCTYPE html>
<html>
  <head>
    <meta http-equiv="content-type" content="text/html; charset=utf-8" />
    <title>Launcher</title>
    <script src="https://code.jquery.com/jquery-3.1.0.min.js" type="text/javascript" charset="utf-8"></script>
    <script type="text/javascript" charset="utf-8">
      $(function() {
        var receiver;

        $('#launch').click(function() {
          if(receiver) { receiver.close(); }

          receiver = window.open('/target.html', 'target', 'width=200,height=200');
        })

        $('#send').click(function() {
          if(receiver) {
            receiver.postMessage("Hello!", "*")
          }
        })
      })
    </script>
  </head>
  <body>
    <button id='launch'>Launch!</button>
    <button id='send'>Send!</button>
  </body>
</html>

## target.html
<!DOCTYPE html>
<html>
  <head>
    <meta http-equiv="content-type" content="text/html; charset=utf-8" />
    <title>Target</title>
    <style>
      p { background: #eee; padding: 5px; font-family: sans-serif; }
    </style>
    <script src="https://code.jquery.com/jquery-3.1.0.min.js" type="text/javascript" charset="utf-8"></script>
    <script type="text/javascript" charset="utf-8">
      var count = 0;
      window.addEventListener("message", function(e) {
        $('body').prepend(
          $('<p>').text(++count + ' ' + e.data)
        );
      });
    </script>
  </head>
  <body>
  </body>
</html>
	<!DOCTYPE html>
	<html>
	<head>
	<meta http-equiv="content-type" content="text/html; charset=utf-8" />
	<title>Launcher</title>
	<script src="https://code.jquery.com/jquery-3.1.0.min.js" type="text/javascript" charset="utf-8"></script>
	<script type="text/javascript" charset="utf-8">
	$(function() {
	var receiver;

	$('#launch').click(function() {
	if(receiver) { receiver.close(); }

	receiver = window.open('/target.html', 'target', 'width=200,height=200');
	})

	$('#send').click(function() {
	if(receiver) {
	receiver.postMessage("Hello!", "*")
	}
	})
	})
	</script>
	</head>
	<body>
	<button id='launch'>Launch!</button>
	<button id='send'>Send!</button>
	</body>
	</html>