Wenting Zhang created 512 (!) icons with just HTML and CSS. Usually just one element, the occasional extra element thrown in, and drawn with things like borders, backgrounds, and box-shadows.
Looks like the same bug bit Wenting that bit Nicolas Gallagher. Taking the shapes that CSS can make and combining them to draw.
Predictably, I’m going to point out that SVG would be a more-ideal tool for this. Less fragile, easier to maintain and style, scaleable, etc. I mention that not to take away from this impressive project, but because I’d love to see a similar project where the icons are drawn with hand-crafted paths and golfed.