Good UI animation serves three purposes: orientation (showing the user what's happening), response (reacting to an action), and mood-setting. The 12 principles of animation (Disney, 1981) — squash and stretch, anticipation, staging, follow through — are still relevant today.
For the web: CSS transitions and animations for simple effects; GSAP for complex animations. Lottie enables After Effects animations in JSON format on both web and mobile. Animation speed: 200–300ms for micro-interactions, 400–600ms for page transitions.
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