js add style file to head code example

Example 1: javascript css link append

document.getElementsByTagName("head")[0].insertAdjacentHTML(
    "beforeend",
    "<link rel=\"stylesheet\" href=\"path/to/style.css\" />");

Example 2: link stylesheet in javascript

var cssId = 'myCss';  // you could encode the css path itself to generate id..
if (!document.getElementById(cssId))
{
    var head  = document.getElementsByTagName('head')[0];
    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';
    head.appendChild(link);
}