Probing for IPv6 support in the browser

If you can read this sentence, you have IPv6 support, either directly or via a web proxy. (Update: no, I added IPv4 access to the blog.)

My home page uses this bit of javascript to keep/remove a sentence depending on whether this page is reachable:

<script language=javascript type="text/javascript">
    x=new XMLHttpRequest();
    x.open( "GET", "https://rant.gulbrandsen.priv.no/ipv6/probe", true );
    x.onreadystatechange = function() {
        if ( x.readyState == 4 ) {
            document.getElementById( "4only" ).innerHTML = ".";
        }
    }
    x.send(null);
</script>

Please use a URL of your own instead of this page.