SO fiddle

Test Fiddle mainly for SO Questions

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8/themes/dot-luv/jquery-ui.css">
<div id="ad_468x60"></div>

<script>
// Safe use of document.write:
document.write("<p><b>This was written before the document was parsed</b></p>");
</script>

JavaScript

// Create a closure to keep the old document.write private 
(function () {
    var oldDW = document.write;
    document.write = function (s) {
        // Document not parsed yet, allow document.write:
        if (document.readyState != "complete")
            oldDW.call(document, s);
        // Dangerous use, switch to innerHTML instead:
        else          
            document.getElementById("ad_468x60").innerHTML = s;
    }
})();


    // Unsafe use of document.write
    window.onload = function () {
        document.write("This was inserted after the document was parsed.  No harm done :-)");
    }