Animation & Motion Skill

This skill covers CSS animations and transitions with a focus on accessibility (respecting user motion preferences) and performance (avoiding jank and layout thrashing).

Related: For CSS-only interactive patterns (tabs, accordions, toggles without JavaScript), see the progressive-enhancement skill.

Philosophy

Motion should be:

1. Purposeful - Guides attention, shows relationships, provides feedback
2. Respectful - Honors prefers-reduced-motion preferences
3. Performant - Uses compositor-only properties when possible
4. Subtle - Enhances, doesn't distract or overwhelm

Reduced Motion First

Always start with reduced motion as the default, then add motion for users who haven't opted out.

The Pattern

/* Base: no motion (reduced motion default) */
.element {
  transition: none;
}

/* Add motion only when user hasn't requested reduced motion */
@media (prefers-reduced-motion: no-preference) {
  .element {
    transition: transform 0.3s ease, opacity 0.3s ease;
  }
}

Why Reduced Motion First?

Respecting User Preferences

The prefers-reduced-motion Media Query

/* User prefers reduced motion */
@media (prefers-reduced-motion: reduce) {
  * {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
  }
}

Granular Reduced Motion

Instead of removing all motion, provide alternatives:

/* Full animation for users without preference */
@media (prefers-reduced-motion: no-preference) {
  .card {
    transition: transform 0.3s ease, box-shadow 0.3s ease;
  }

  .card:hover {
    transform: translateY(-4px);
    box-shadow: var(--shadow-lg);
  }
}

/* Subtle alternative for reduced motion */
@media (prefers-reduced-motion: reduce) {
  .card {
    transition: box-shadow 0.15s ease;
  }

  .card:hover {
    box-shadow: var(--shadow-md);
  }
}

JavaScript Detection

const prefersReducedMotion = window.matchMedia(
  '(prefers-reduced-motion: reduce)'
).matches;

if (prefersReducedMotion) {
  // Use instant transitions or skip animations
  element.style.transition = 'none';
} else {
  // Full animation
  element.animate(keyframes, options);
}

Performance-Safe Properties

Compositor-Only Properties (Fast)

These properties can be animated without triggering layout or paint:

/* GOOD: Compositor-only */
.card:hover {
  transform: translateY(-4px) scale(1.02);
  opacity: 0.9;
}

Properties to Avoid Animating

These trigger expensive layout recalculations:

/* BAD: Triggers layout */
.card:hover {
  margin-top: -4px;  /* Layout thrashing */
  height: 110%;      /* Layout thrashing */
}

/* GOOD: Use transform instead */
.card:hover {
  transform: translateY(-4px) scaleY(1.1);
}

Promoting to Compositor Layer

Use will-change sparingly for known animations:

/* Only use for elements that WILL animate */
.animated-element {
  will-change: transform, opacity;
}

/* Remove after animation completes */
.animated-element.animation-done {
  will-change: auto;
}

Warning: Don't apply will-change to many elements—it consumes memory.

Transition Patterns

Design Token Integration

:root {
  /* Duration scale */
  --duration-instant: 0.1s;
  --duration-fast: 0.15s;
  --duration-normal: 0.3s;
  --duration-slow: 0.5s;

  /* Easing functions */
  --ease-out: cubic-bezier(0, 0, 0.2, 1);
  --ease-in: cubic-bezier(0.4, 0, 1, 1);
  --ease-in-out: cubic-bezier(0.4, 0, 0.2, 1);
  --ease-bounce: cubic-bezier(0.68, -0.55, 0.265, 1.55);
}

Common Transitions

/* Hover lift effect */
@media (prefers-reduced-motion: no-preference) {
  .card {
    transition:
      transform var(--duration-fast) var(--ease-out),
      box-shadow var(--duration-fast) var(--ease-out);
  }

  .card:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
  }
}

/* Focus ring */
@media (prefers-reduced-motion: no-preference) {
  button {
    transition: outline-offset var(--duration-instant) var(--ease-out);
  }

  button:focus-visible {
    outline: 2px solid var(--focus-color);
    outline-offset: 2px;
  }
}

/* Fade in */
@media (prefers-reduced-motion: no-preference) {
  .fade-in {
    animation: fadeIn var(--duration-normal) var(--ease-out);
  }
}

@keyframes fadeIn {
  from { opacity: 0; }
  to { opacity: 1; }
}

Animation Patterns

Keyframe Animations

/* Subtle pulse for attention */
@keyframes pulse {
  0%, 100% { transform: scale(1); }
  50% { transform: scale(1.05); }
}

@media (prefers-reduced-motion: no-preference) {
  .notification-badge {
    animation: pulse 2s var(--ease-in-out) infinite;
  }
}

/* Reduced motion alternative: no animation */
@media (prefers-reduced-motion: reduce) {
  .notification-badge {
    animation: none;
  }
}

Loading Spinners

/* Spinner that respects reduced motion */
@keyframes spin {
  to { transform: rotate(360deg); }
}

.spinner {
  width: 24px;
  height: 24px;
  border: 2px solid var(--border-color);
  border-top-color: var(--primary-color);
  border-radius: 50%;
}

@media (prefers-reduced-motion: no-preference) {
  .spinner {
    animation: spin 1s linear infinite;
  }
}

@media (prefers-reduced-motion: reduce) {
  .spinner {
    /* Static indicator or pulsing opacity */
    animation: none;
    border-style: dotted;
  }
}

Skeleton Loading

@keyframes shimmer {
  0% { background-position: -200% 0; }
  100% { background-position: 200% 0; }
}

.skeleton {
  background: linear-gradient(
    90deg,
    var(--surface-color) 25%,
    var(--background-alt) 50%,
    var(--surface-color) 75%
  );
  background-size: 200% 100%;
}

@media (prefers-reduced-motion: no-preference) {
  .skeleton {
    animation: shimmer 1.5s infinite;
  }
}

@media (prefers-reduced-motion: reduce) {
  .skeleton {
    animation: none;
    background: var(--background-alt);
  }
}

Entrance Animations

Fade and Slide

@keyframes fadeSlideUp {
  from {
    opacity: 0;
    transform: translateY(10px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

@media (prefers-reduced-motion: no-preference) {
  .animate-in {
    animation: fadeSlideUp var(--duration-normal) var(--ease-out) both;
  }

  /* Staggered children */
  .animate-in > * {
    animation: fadeSlideUp var(--duration-normal) var(--ease-out) both;
  }

  .animate-in > *:nth-child(1) { animation-delay: 0ms; }
  .animate-in > *:nth-child(2) { animation-delay: 50ms; }
  .animate-in > *:nth-child(3) { animation-delay: 100ms; }
  .animate-in > *:nth-child(4) { animation-delay: 150ms; }
}

@media (prefers-reduced-motion: reduce) {
  .animate-in,
  .animate-in > * {
    animation: none;
    opacity: 1;
    transform: none;
  }
}

Scale In

@keyframes scaleIn {
  from {
    opacity: 0;
    transform: scale(0.9);
  }
  to {
    opacity: 1;
    transform: scale(1);
  }
}

@media (prefers-reduced-motion: no-preference) {
  dialog[open] {
    animation: scaleIn var(--duration-fast) var(--ease-out);
  }
}

View Transitions API

For page transitions (progressive enhancement):

/* Enable view transitions */
@view-transition {
  navigation: auto;
}

/* Default crossfade */
::view-transition-old(root),
::view-transition-new(root) {
  animation-duration: var(--duration-normal);
}

/* Respect reduced motion */
@media (prefers-reduced-motion: reduce) {
  ::view-transition-old(root),
  ::view-transition-new(root) {
    animation-duration: 0.01ms;
  }
}

/* Named transitions for specific elements */
.hero-image {
  view-transition-name: hero;
}

::view-transition-old(hero),
::view-transition-new(hero) {
  animation-duration: var(--duration-slow);
}

Scroll-Driven Animations

Modern CSS scroll-driven animations (progressive enhancement):

/* Fade in on scroll */
@keyframes fadeInOnScroll {
  from { opacity: 0; transform: translateY(20px); }
  to { opacity: 1; transform: translateY(0); }
}

@media (prefers-reduced-motion: no-preference) {
  .scroll-reveal {
    animation: fadeInOnScroll linear both;
    animation-timeline: view();
    animation-range: entry 0% entry 100%;
  }
}

@media (prefers-reduced-motion: reduce) {
  .scroll-reveal {
    opacity: 1;
    transform: none;
  }
}

Micro-interactions

Button Press

@media (prefers-reduced-motion: no-preference) {
  button {
    transition: transform var(--duration-instant) var(--ease-out);
  }

  button:active {
    transform: scale(0.98);
  }
}

Toggle Switch

.toggle-track {
  width: 44px;
  height: 24px;
  background: var(--surface-color);
  border-radius: 12px;
}

.toggle-thumb {
  width: 20px;
  height: 20px;
  background: white;
  border-radius: 50%;
  transform: translateX(2px);
}

@media (prefers-reduced-motion: no-preference) {
  .toggle-thumb {
    transition: transform var(--duration-fast) var(--ease-out);
  }
}

.toggle-input:checked + .toggle-track .toggle-thumb {
  transform: translateX(22px);
}

Checkbox Check

.checkbox-icon {
  stroke-dasharray: 24;
  stroke-dashoffset: 24;
}

@media (prefers-reduced-motion: no-preference) {
  .checkbox-icon {
    transition: stroke-dashoffset var(--duration-fast) var(--ease-out);
  }
}

.checkbox-input:checked + .checkbox-box .checkbox-icon {
  stroke-dashoffset: 0;
}

Animation Duration Guidelines

The 100ms Rule

Users perceive actions as instant if response is under 100ms. Use this for:

• Button active states
• Focus indicators
• Toggle switches

Dangerous Patterns to Avoid

Vestibular Triggers

These can cause motion sickness or seizures:

/* BAD: Aggressive parallax */
.parallax {
  transform: translateY(calc(var(--scroll) * 0.5));
}

/* BETTER: Subtle or disabled with reduced motion */
@media (prefers-reduced-motion: reduce) {
  .parallax {
    transform: none;
  }
}

Infinite Animations

/* BAD: Constant motion */
.attention-seeker {
  animation: bounce 1s infinite;
}

/* BETTER: Limited iterations */
.attention-seeker {
  animation: bounce 1s 3; /* Only 3 times */
}

/* BEST: Trigger on interaction */
.attention-seeker:hover {
  animation: bounce 0.5s;
}

Testing Checklist

Browser DevTools

1. Chrome: Rendering > Emulate CSS media feature > prefers-reduced-motion: reduce
2. Firefox: about:config > ui.prefersReducedMotion (0=no-preference, 1=reduce)
3. Safari: Develop > Experimental Features > Reduced Motion

System Settings

• macOS: System Preferences > Accessibility > Display > Reduce motion
• iOS: Settings > Accessibility > Motion > Reduce Motion
• Windows: Settings > Ease of Access > Display > Show animations
• Android: Settings > Accessibility > Remove animations

Checklist

When adding animations or transitions:

• prefers-reduced-motion is respected
• Reduced motion has a meaningful alternative (not just disabled)
• Only compositor properties are animated (transform, opacity)
• will-change is used sparingly and removed after animation
• Duration tokens are used consistently
• No flashing content (>3 flashes per second)
• Infinite animations have a purpose and can be stopped
• Parallax and large motion are optional enhancements
• Loading states work without animation
• Animation enhances rather than distracts

Related Skills

• css-author - Modern CSS organization with native @import, @layer casca...
• progressive-enhancement - HTML-first development with CSS-only interactivity patterns
• performance - Write performance-friendly HTML pages
• accessibility-checker - Ensure WCAG2AA accessibility compliance