CSS Animations: A Complete Guide to Keyframes, Transitions, and Performance

CSS Transitions vs Animations

CSS offers two ways to animate elements:

Transitions — Animate between two states when a property changes (hover, focus, class toggle) Animations — Define multi-step animations with keyframes that can run automatically

When to Use Each

Feature	Transitions	Animations
Trigger	Property change	Automatic or class
States	2 (start → end)	Unlimited (keyframes)
Looping	No	Yes
Complexity	Simple	Complex
Best for	Hover effects, toggles	Loading spinners, attention

CSS Transitions

.button {
  background: #6366f1;
  transform: scale(1);
  transition: all 0.3s ease;
}

.button:hover {
  background: #4f46e5;
  transform: scale(1.05);
}

Transition Properties

/* Shorthand */
transition: property duration timing-function delay;

/* Individual properties */
transition-property: transform, opacity;
transition-duration: 0.3s;
transition-timing-function: ease-in-out;
transition-delay: 0.1s;

/* Multiple transitions */
transition:
  transform 0.3s ease,
  opacity 0.2s ease 0.1s,
  background-color 0.3s ease;

Timing Functions

transition-timing-function: ease;        /* Default — slow start, fast middle, slow end */
transition-timing-function: linear;      /* Constant speed */
transition-timing-function: ease-in;     /* Slow start */
transition-timing-function: ease-out;    /* Slow end */
transition-timing-function: ease-in-out; /* Slow start and end */
transition-timing-function: cubic-bezier(0.68, -0.55, 0.265, 1.55); /* Custom bounce */

CSS Keyframe Animations

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

.element {
  animation: fadeIn 0.5s ease-out;
}

Multi-Step Animations

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

@keyframes rainbow {
  0% { color: red; }
  17% { color: orange; }
  33% { color: yellow; }
  50% { color: green; }
  67% { color: blue; }
  83% { color: indigo; }
  100% { color: violet; }
}

Animation Properties

/* Shorthand */
animation: name duration timing-function delay iteration-count direction fill-mode;

/* Individual */
animation-name: fadeIn;
animation-duration: 0.5s;
animation-timing-function: ease-out;
animation-delay: 0.2s;
animation-iteration-count: infinite; /* or a number */
animation-direction: alternate;      /* normal, reverse, alternate, alternate-reverse */
animation-fill-mode: forwards;       /* none, forwards, backwards, both */
animation-play-state: running;       /* running, paused */

Practical Animation Examples

Fade In on Scroll

.fade-in {
  opacity: 0;
  transform: translateY(30px);
  transition: opacity 0.6s ease, transform 0.6s ease;
}

.fade-in.visible {
  opacity: 1;
  transform: translateY(0);
}

Loading Spinner

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

.spinner {
  width: 40px;
  height: 40px;
  border: 3px solid #333;
  border-top-color: #6366f1;
  border-radius: 50%;
  animation: spin 0.8s linear infinite;
}

Skeleton Loading

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

.skeleton {
  background: linear-gradient(90deg, #1a1a2e 25%, #2a2a4e 50%, #1a1a2e 75%);
  background-size: 200% 100%;
  animation: shimmer 1.5s infinite;
  border-radius: 4px;
}

Bounce Effect

@keyframes bounce {
  0%, 20%, 50%, 80%, 100% { transform: translateY(0); }
  40% { transform: translateY(-20px); }
  60% { transform: translateY(-10px); }
}

.bounce { animation: bounce 1s ease infinite; }

Performance Optimization

The Golden Rule

Only animate these properties for smooth 60fps animations:

• transform (translate, rotate, scale)
• opacity

These are handled by the GPU compositor and don't trigger layout or paint.

Properties to AVOID Animating

/* ❌ Triggers layout (very expensive) */
width, height, margin, padding, top, left, right, bottom, font-size

/* ❌ Triggers paint (expensive) */
background-color, color, border-color, box-shadow, border-radius

/* ✅ Compositor only (cheap) */
transform, opacity, filter

Instead of Animating Width/Height

/* ❌ Bad — animates width */
.element { width: 100px; transition: width 0.3s; }
.element:hover { width: 200px; }

/* ✅ Good — animates transform */
.element { transform: scaleX(1); transition: transform 0.3s; }
.element:hover { transform: scaleX(2); }

will-change

/* Tell the browser to optimize for upcoming animation */
.will-animate {
  will-change: transform, opacity;
}

/* Remove after animation completes to free resources */

prefers-reduced-motion

Always respect user preferences:

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

Design your animations visually with our CSS Animation Generator tool.

Related Tools

• CSS Animation Generator — Create keyframe animations visually
• CSS Gradient Generator — Animated gradient backgrounds
• CSS Box Shadow Generator — Shadow hover effects
• CSS Flexbox Playground — Layout for animated elements
• Color Converter — Convert colors for animations
• CSS Minifier — Minify animation CSS for production