Basically, Nicolas is using css pseudo elements (:before and :after) and semantic HTML to create a 84 icons set. Let me immediately warn you: with Internet Explorer you won't see things as they were conceived. If you open the demo page in IE8, for example, you will see the icons with some css attributes missing and the whole effect will be missed.
I suggest to visit Nicolas page and have a look at his works. Just keep in mind that pure css GUI icons are just an experiment.