I would like to create a page that runs a 3rd party script that includes document.write after the DOM was already fully loaded.
My page is not XHTML. My
In order to alter the content of the page after the DOM has rendered you need to either use a javascript library to append HTML or text at certain points (jQuery, mootools, prototype, ...) or just use the innerHTML property of each DOM element to alter/append text to it. This works crossbrowser and doesn't require any libraries.