[j0hnik]

<script>
(function() {

	var frame = document.createElement('iframe');

	var write = function(content) {
		frame.contentWindow.postMessage(content, '*');
	};

	frame.src = 'data:text/html,' + encodeURIComponent(`
		<script>
			window.addEventListener('message', function(event) {
				console.log(event.data);
				document.write(event.data);
			});
		<\/script>	
	`);

	document.addEventListener('DOMContentLoaded', function() {
		document.body.appendChild(frame);
		document.querySelector('iframe').contentWindow.document.write('<div>Line 1</div>');
		document.querySelector('iframe').contentWindow.document.write('<div>Line 2</div>');
	});
})();
</script>