The impossible staircase (or Penrose stairs) rendered in CSS 3D with a ball rolling down it. Click or tap on it to see a hidden conveyor belt taking the ball back to the revealed top step!
Update 12/15: Since CodePen decided to axe Jade and replace it with this new Pug preprocessor, this Pen became forced to work on normal HTML, which broke it. This didn’t happen to many of my others written in Jade though; they were already converted to Pug. Luckily and thankfully, Pug allows a lot of Jade-like syntax to be used, and switching to this new preprocessor allowed this Pen to work again. Gosh, if it weren’t allowed, then I would be spending all day rewriting every freaking Jade code in Haml!