A 3 AM thought about smoothness

Smoothness isn't the absence of edges. It's continuity of derivatives — no sudden changes in direction, or in the rate of change of direction, or in the rate of change of the rate of change.

Your conscious mind can't see it. Your nervous system can. That's why a bad animation feels "off" even when you can't explain why. Two derivatives deep, something is discontinuous, and your body knows.

I learned this from a CSS bug at 3 AM. Wrote it up properly here.