Efficient Animated Content

Why It Matters

• Inefficient animations cause jank. When animations trigger expensive layout recalculations or repaints, the browser drops frames. What should be smooth motion turns into visible stuttering — especially on mobile devices.
• They drain device batteries. Animations that constantly recompute layout or repaint large areas keep the CPU and GPU running at high utilization. On laptops and phones, this translates directly to shorter battery life.
• Performance affects user perception. A slow, laggy animation makes the entire site feel broken — even if the content is excellent. Users judge site quality by how fluid interactions feel, not just how fast pages load.
• Heavy animations block the main thread. Animations that require JavaScript or layout recalculation on every frame compete with other interactive features for CPU time. Scrolling, clicking, and typing can become unresponsive.

How to Animate Efficiently

1. Use transform and opacity for animations. These two CSS properties can be animated by the GPU without triggering layout or repaint. Instead of animating width, height, or top, use transform: translateX(), scale(), or opacity.
2. Use CSS animations over JavaScript when possible. CSS animations and transitions are optimized by the browser and can run on the compositor thread — off the main thread entirely. This keeps animations smooth even when JavaScript is busy.
3. Avoid animating layout-triggering properties. Properties like width, height, margin, padding, and top/left force the browser to recalculate layout on every frame. This is expensive and causes jank.
4. Use will-change sparingly. The will-change CSS property can hint to the browser that an element is about to be animated, allowing it to optimize in advance. But overusing it wastes memory — apply it only to elements that are actually about to animate.
5. Replace animated GIFs with video. Animated GIF files are enormous and inefficient. A short looping video in MP4 or WebM format delivers the same visual at a fraction of the file size and with hardware-accelerated playback.

Common Mistakes

• Animating left and top instead of transform. Moving an element by changing its left or top position triggers layout recalculation on every frame. Use transform: translate() instead — it is GPU-accelerated and avoids layout thrashing.
• Using large animated GIFs. A 5-second animated GIF can easily be 10MB or more. The same animation as a compressed video might be 200KB. Always use video formats for animated content longer than a second or two.
• Ignoring prefers-reduced-motion. Some users have motion sensitivity. Always check for the prefers-reduced-motion media query and reduce or remove animations for users who request it.
• Running animations when not visible. Animations playing off-screen still consume resources. Use the Intersection Observer API to pause animations when elements leave the viewport.