Animation Quality

Animation Quality

Purpose

All animations stay on the compositor thread (60fps), respect user motion preferences, and use the right tool (CSS vs JS) for the job.

Universal — GPU-compositor rules (transform/opacity only), prefers-reduced-motion compliance, CSS-vs-JS choice, and 60fps target are browser-level concerns identical across frameworks.

Procedure

1. Audit animated properties — restrict to compositor-friendly props

   • First, does each animation earn its place? Motion should communicate state change, spatial continuity, or feedback — cut decorative motion that only delays the user.
   • Allowed: transform (translate/scale/rotate/skew), opacity, filter
   • Banned: top, left, width, height, margin, padding (trigger layout/paint)
   • Animating layout also shifts surrounding content → CLS. Entrance/expand animations (accordions, height: auto) are the classic offender — reserve the space, or animate transform / grid-template-rows: 0fr→1fr instead of height
   • Search: grep -rE 'transition.*?(width|height|top|left|margin|padding)' src/

2. Respect prefers-reduced-motion globally

   • Single source of truth at the app root — opt the whole app into the user's motion preference. (see Implementation; Framer Motion)
   • Scope clarification: the global "user-preference" mode disables transform and layout animations while preserving opacity and color transitions — not all motion is killed. This is the intended behavior (preserves accessibility-safe feedback).
   • Allow per-component overrides where a critical affordance must still move (e.g., an onboarding cue): detect the preference and conditionally adjust animation props
   • Cover decorative animations outside the motion library too (e.g., a CSS-level reduced-motion variant — see Implementation)
   • Auto-playing or looping motion that runs >5s alongside other content must be pausable/stoppable — carousels, looping backgrounds, marquees (WCAG 2.2.2)
   • Nothing flashes more than 3×/second — avoid rapid flash/strobe effects entirely (WCAG 2.3.1, seizure risk)
   • Verify in OS accessibility settings (e.g., macOS System Preferences → Accessibility → Display → Reduce motion)

3. Audit will-change usage

   • will-change should be applied just before animation, removed after
   • Permanent will-change on many elements degrades performance
   • Use sparingly; default is no will-change

4. Choose the right tool — and tune the feel

   • CSS transition: simple state changes (hover, focus, open/close)
   • Framer Motion / Motion: physics, gestures, complex sequences, layout animations
   • Duration: micro-interactions 100–200ms, most UI transitions 200–300ms, larger/page transitions ≤ ~500ms — too-slow motion feels sluggish and hurts perceived performance
   • Easing: ease-out for entrances (decelerate in), ease-in for exits, ease-in-out for moves; avoid linear for UI motion (reserve it for continuous/looping)

5. Use the animation library's primitives correctly

   • Animate unmount/exit transitions — easy to forget
   • Prefer the lib's layout-animation over manual position math for size/position changes
   • Ensure shared-element transitions have no orphans (every entry has a matching exit). (see Implementation; Framer AnimatePresence / layout / layoutId)

6. Scroll-driven animations

   • Prefer IntersectionObserver or Motion's useScroll
   • Avoid scroll event listeners (run on main thread)

7. JS animations use requestAnimationFrame

   • Never setTimeout for animation loops
   • Cancel rAF on cleanup

8. Verify (validation loop)

   • Chrome DevTools → Performance: record the interaction; look for dropped frames, layout thrashing, paint regions
   • If not 60fps / 0 dropped frames, find the non-compositor property or main-thread work and return to step 1 / step 7
   • Confirm interactions stay responsive during animation (INP unaffected — heavy JS animation on the main thread blocks input)
   • Toggle OS "Reduce motion" and confirm the app responds; confirm nothing flashes >3×/s
   • Loop until: 60fps + 0 dropped frames + reduced-motion honored

Severity tiers

Completion Criteria

• Animated properties limited to transform/opacity/filter
• prefers-reduced-motion honored globally
• No content flashes >3×/s; auto-playing motion >5s is pausable (WCAG 2.3.1 / 2.2.2)
• No animation shifts surrounding layout (no CLS from motion)
• Dropped frames = 0 in DevTools Performance trace
• No will-change permanent declarations
• AnimatePresence covers all conditional renders with exit animations

Output

• Animation code: refactored to compositor-only properties (transform / opacity / filter)
• Reduced-motion config: global <MotionConfig reducedMotion="user"> (or framework equivalent) wired at app root
• Performance trace: Chrome DevTools Performance recording showing 60fps + 0 dropped frames, saved as docs/perf-trace-YYYY-MM-DD.json
• Animation inventory (paste into PR): list of animations with type (decorative / functional / scroll-driven) and library used
• Commit format: fix(anim): switch <component> to transform/opacity for fixes; feat(anim): add <interaction> for new

Implementation

React + Next.js (default)

• Library: Motion (formerly Framer Motion) — <MotionConfig reducedMotion="user">, useReducedMotion() hook, <AnimatePresence>, layout + layoutId
• Tailwind reduced-motion variant: motion-reduce:transition-none
• CSS transitions for simple state changes; Motion for physics / gestures / layout

Other stacks

• Vue / Nuxt: <Transition> / <TransitionGroup> built-in; @vueuse/motion for spring physics; prefers-reduced-motion via Tailwind variant or CSS
• SvelteKit: built-in transition: directive (fade, fly, scale, crossfade) + motion package; prefers-reduced-motion store available via @svelte-put/preferences
• Angular: @angular/animations module with trigger(), state(), animate(); BreakpointObserver for reduced-motion media query
• Universal: GPU-compositor rules (transform/opacity only, avoid top/left/width/height) work in every browser; prefers-reduced-motion is a CSS media query that any framework can detect; Chrome DevTools Performance panel measures dropped frames framework-agnostically

Related skills

• rendering-performance — animation jank often manifests as INP regression
• accessibility-audit — reduced-motion respect is a WCAG requirement

Reference

• Key insight encoded: Wrap the app in <MotionConfig reducedMotion="user"> for global opt-out — single source of truth. Constrain animated props to transform/opacity so animations stay on the GPU (and don't shift layout → CLS). Prefer layout over manual position animation for free FLIP-style transitions. Motion-specific WCAG lives in this skill, not just reduced-motion: nothing flashes >3×/s (2.3.1), and auto-playing motion >5s must be pausable (2.2.2). Tune the feel — fast durations (100–300ms) with ease-out entrances / ease-in exits.