I know how to write into an iframe:

Code:
<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Writer</title>
    <style>
        textarea,
        iframe {
            display: block;
            width: 800px;
            height: 200px;
        }
    </style>
</head>

<body>
    <textarea id="ta" oninput="writeIt();"></textarea>
    <iframe id="frm"></iframe>
    <script>
        function writeIt() {
            var ta = document.getElementById('ta');
            var frm = document.getElementById('frm');
            var frmDoc = frm.contentDocument;
            frmDoc.open();
            frmDoc.write(ta.value);
            frmDoc.close();
        }
    </script>
</body>

</html>
DEMO

There's a problem, though. Enter the following into the textarea:

<a href="http://www.example.com/">Example.com</a>

Now click on the link, and then get back to the textarea to continue writing. It doesn't work anymore due to the cross-origin restriction. Is there any way in this case to continue writing?