Using document.write like you did is bad because fetching resources asynchronously prevents those resources from blocking the page load.

Using a script DOM element maximizes asynchronous loading across current browsers:

<script>
var node = document.createElement(‘link’);
node.type = ‘text/css’;
node.async = true;
node.src = ‘example.css’;
// Now insert the node into the DOM, perhaps using insertBefore()
</script>