CSS Animations

Native browser animations - GPU-accelerated, no dependencies, excellent performance.

Quick Start

/* Define keyframes */
@keyframes fadeIn {
  from {
    opacity: 0;
    transform: translateY(20px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

/* Apply animation */
.element {
  animation: fadeIn 0.5s ease-out forwards;
}

Transitions vs Animations

Transitions	Animations
Two states only	Multiple keyframes
Triggered by state change	Can auto-play
Simpler syntax	More control
:hover, :focus, class change	Loops, delays, direction

CSS Transitions

For simple two-state animations.

.button {
  background: blue;
  transform: scale(1);
  transition: all 0.3s ease-out;

  /* Or be specific (better performance) */
  transition:
    background 0.3s ease-out,
    transform 0.2s ease-out;
}

.button:hover {
  background: darkblue;
  transform: scale(1.05);
}

Transition Properties

.element {
  transition-property: transform, opacity;
  transition-duration: 0.3s;
  transition-timing-function: ease-out;
  transition-delay: 0.1s;

  /* Shorthand: property duration timing-function delay */
  transition: transform 0.3s ease-out 0.1s;
}

@keyframes Syntax

/* Using from/to */
@keyframes slide {
  from {
    transform: translateX(-100%);
  }
  to {
    transform: translateX(0);
  }
}

/* Using percentages */
@keyframes bounce {
  0% {
    transform: translateY(0);
  }
  50% {
    transform: translateY(-30px);
  }
  100% {
    transform: translateY(0);
  }
}

/* Multiple properties at same point */
@keyframes complex {
  0%, 100% {
    transform: scale(1);
    opacity: 1;
  }
  50% {
    transform: scale(1.2);
    opacity: 0.8;
  }
}

Animation Properties

.element {
  /* Required */
  animation-name: fadeIn;
  animation-duration: 0.5s;

  /* Optional */
  animation-timing-function: ease-out;
  animation-delay: 0.2s;
  animation-iteration-count: 1;       /* or infinite */
  animation-direction: normal;
  animation-fill-mode: forwards;
  animation-play-state: running;

  /* Shorthand */
  animation: fadeIn 0.5s ease-out 0.2s 1 normal forwards running;

  /* Multiple animations */
  animation:
    fadeIn 0.5s ease-out,
    slideUp 0.5s ease-out 0.1s;
}

Animation Property Values

Property	Values
animation-timing-function	linear, ease, ease-in, ease-out, ease-in-out, cubic-bezier(), steps()
animation-iteration-count	1, 2, 3, infinite
animation-direction	normal, reverse, alternate, alternate-reverse
animation-fill-mode	none, forwards, backwards, both
animation-play-state	running, paused

Fill Mode Explained

/* none - Returns to original state after animation */
animation-fill-mode: none;

/* forwards - Keeps final keyframe state */
animation-fill-mode: forwards;

/* backwards - Applies first keyframe during delay */
animation-fill-mode: backwards;

/* both - forwards + backwards */
animation-fill-mode: both;

Timing Functions

/* Built-in */
animation-timing-function: linear;
animation-timing-function: ease;        /* default */
animation-timing-function: ease-in;
animation-timing-function: ease-out;
animation-timing-function: ease-in-out;

/* Custom cubic bezier */
animation-timing-function: cubic-bezier(0.68, -0.55, 0.265, 1.55);

/* Steps (for sprite animations) */
animation-timing-function: steps(6);
animation-timing-function: steps(6, start);
animation-timing-function: steps(6, end);

Common Cubic Bezier Values

/* Smooth deceleration */
cubic-bezier(0, 0, 0.2, 1)

/* Smooth acceleration */
cubic-bezier(0.4, 0, 1, 1)

/* Standard ease */
cubic-bezier(0.4, 0, 0.2, 1)

/* Overshoot (bouncy) */
cubic-bezier(0.68, -0.55, 0.265, 1.55)

/* Elastic */
cubic-bezier(0.68, -0.6, 0.32, 1.6)

Performance Best Practices

GPU-Accelerated Properties (Composite Only)

/* GOOD - GPU accelerated, no layout/paint */
transform: translateX(100px);
transform: scale(1.5);
transform: rotate(45deg);
opacity: 0.5;

/* Hint for GPU acceleration */
will-change: transform, opacity;

Avoid Animating (Cause Reflow/Repaint)

/* BAD - triggers layout recalculation */
width, height, top, left, right, bottom,
margin, padding, border-width,
font-size, line-height

Use Transform Instead

/* Instead of: left: 100px */
transform: translateX(100px);

/* Instead of: width: 200px */
transform: scaleX(2);

Common Animation Patterns

Fade In

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

.fade-in {
  animation: fadeIn 0.3s ease-out forwards;
}

Slide In From Bottom

@keyframes slideUp {
  from {
    opacity: 0;
    transform: translateY(30px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

.slide-up {
  animation: slideUp 0.5s ease-out forwards;
}

Scale In

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

Pulse

@keyframes pulse {
  0%, 100% {
    transform: scale(1);
  }
  50% {
    transform: scale(1.05);
  }
}

.pulse {
  animation: pulse 2s ease-in-out infinite;
}

Shake

@keyframes shake {
  0%, 100% { transform: translateX(0); }
  10%, 30%, 50%, 70%, 90% { transform: translateX(-5px); }
  20%, 40%, 60%, 80% { transform: translateX(5px); }
}

.shake {
  animation: shake 0.5s ease-in-out;
}

Spin

@keyframes spin {
  from { transform: rotate(0deg); }
  to { transform: rotate(360deg); }
}

.spinner {
  animation: spin 1s linear infinite;
}

Bounce

@keyframes bounce {
  0%, 100% {
    transform: translateY(0);
    animation-timing-function: cubic-bezier(0, 0, 0.2, 1);
  }
  50% {
    transform: translateY(-25%);
    animation-timing-function: cubic-bezier(0.8, 0, 1, 1);
  }
}

.bounce {
  animation: bounce 1s infinite;
}

Skeleton Loading

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

.skeleton {
  background: linear-gradient(
    90deg,
    #f0f0f0 25%,
    #e0e0e0 50%,
    #f0f0f0 75%
  );
  background-size: 200% 100%;
  animation: shimmer 1.5s infinite;
}

Accessibility

Respect Reduced Motion

/* Define full animation */
.element {
  animation: fadeIn 0.5s ease-out forwards;
}

/* Reduce or remove for users who prefer reduced motion */
@media (prefers-reduced-motion: reduce) {
  .element {
    animation: none;
    opacity: 1;
  }

  /* Or use simpler animation */
  *,
  *::before,
  *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
  }
}

Pause on Hover (for continuous animations)

.element {
  animation: spin 2s linear infinite;
}

.element:hover {
  animation-play-state: paused;
}

JavaScript Control

// Add animation class
element.classList.add('animate-in');

// Remove after animation ends
element.addEventListener('animationend', () => {
  element.classList.remove('animate-in');
});

// Restart animation
function restartAnimation(element) {
  element.style.animation = 'none';
  element.offsetHeight; // Trigger reflow
  element.style.animation = null;
}

// Pause/resume
element.style.animationPlayState = 'paused';
element.style.animationPlayState = 'running';

Animation Events

element.addEventListener('animationstart', (e) => {
  console.log('Started:', e.animationName);
});

element.addEventListener('animationiteration', (e) => {
  console.log('Iteration:', e.elapsedTime);
});

element.addEventListener('animationend', (e) => {
  console.log('Ended:', e.animationName);
});

element.addEventListener('animationcancel', (e) => {
  console.log('Cancelled');
});

Staggered Animations

.item { animation: fadeIn 0.5s ease-out forwards; opacity: 0; }
.item:nth-child(1) { animation-delay: 0.1s; }
.item:nth-child(2) { animation-delay: 0.2s; }
.item:nth-child(3) { animation-delay: 0.3s; }
.item:nth-child(4) { animation-delay: 0.4s; }
.item:nth-child(5) { animation-delay: 0.5s; }

/* With CSS custom properties */
.item {
  animation: fadeIn 0.5s ease-out forwards;
  animation-delay: calc(var(--index) * 0.1s);
  opacity: 0;
}

<div class="item" style="--index: 0">Item 1</div>
<div class="item" style="--index: 1">Item 2</div>
<div class="item" style="--index: 2">Item 3</div>

CSS Custom Properties for Dynamic Animations

:root {
  --animation-duration: 0.3s;
  --animation-easing: cubic-bezier(0.4, 0, 0.2, 1);
}

.element {
  transition: transform var(--animation-duration) var(--animation-easing);
}

/* Override per component */
.modal {
  --animation-duration: 0.5s;
}

Reference Files

• references/keyframe-patterns.md - Common animation patterns library