Here's the "oldschool" way of doing it, which hopefully works accross all browsers. In theory you would use setAttribute unfortunately IE6 doesn't support it consistently
var cssId = 'myCss'; // you could encode the css path itself to generate id..
var head = document.getElementsByTagName('head');
var link = document.createElement('link');
link.id = cssId;
link.rel = 'stylesheet';
link.type = 'text/css';
link.href = 'http://website.com/css/stylesheet.css';
link.media = 'all';
This example checks if the CSS was already added so it adds it only once.