Accessibility
Respect prefers-reduced-motion: every animation needs a reduced variant, usually a fast fade or nothing at all. And motion must never be the only carrier of information — pair it with a state that persists, like a color, icon, or label change.
A checklist before shipping any animation
Does it give feedback, orient, or direct focus — or is it decoration? Will users see it twenty times a day? Is it keyboard-triggered? Is it under 300ms with the right easing? Does its motion depict where the element comes from? Does it only touch transform and opacity, and can it be interrupted? Does it degrade under reduced motion?
If an animation survives all of that, ship it. Most won’t — and that’s the point. The product will feel faster and more polished with ten deliberate animations than with a hundred reflexive ones.