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.
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.