To take this to a fun little limit, I tried to turn it into a self-saving form. Unfortunately, I hit problems with the History API being forbidden, replacing the URL seemed to only reload the page and not change it and so on... But I did eventually get something that is quite useable, which you should obviously not copy and past blindly:

    data:text/html,<div id="c" contenteditable></div><a id="p"></a><script>var el=document.getElementById('c');el.addEventListener('input',function(){document.getElementById('p').textContent='Copy me';document.getElementById('p').href="data:text/html,"+this.parentElement.innerHTML;},false);</script>

And to expand that awful payload, it's basically the very simple:

    <div id="c" contenteditable></div>
    <a id="p"></a>
    <script>
    var el = document.getElementById('c');
    el.addEventListener('input', function() {
        document.getElementById('p').textContent = 'Copy me';
        document.getElementById('p').href = "data:text/html," + this.parentElement.innerHTML;
    }, false);
    </script>

The idea being, you can _almost_ have a self-saving editable page. Because it was fun to make.