var doc = document; // shortcut
var cssId = 'myCss';  // you could encode the css path itself to generate id..
if (!doc.getElementById(cssId))
{
    var head  = doc.getElementsByTagName('head')[0];
    var link  = doc.createElement('link');
    link.id   = cssId;
    link.rel  = 'stylesheet';
    link.type = 'text/css';
    link.href = '/externalModules/fancybox/public/jquery.fancybox-1.3.1.css';
    link.media = 'all';
    head.appendChild(link);
}