How do CSS keyframe animations work?

Question

Accepted Answer

While transitions animate *between two states*, **`@keyframes` animations** define a multi-step sequence that can run automatically, loop, and move through many intermediate states.

## Defining and applying an animation

```css
@keyframes slide-in {
  from { transform: translateX(-100%); opacity: 0; } /* 0% */
  to   { transform: translateX(0);     opacity: 1; } /* 100% */
}

.panel {
  animation: slide-in 0.5s ease-out;
}
```

## Multi-step keyframes with percentages

```css
@keyframes bounce {
  0%   { transform: translateY(0); }
  50%  { transform: translateY(-30px); } /* peak of the bounce */
  100% { transform: translateY(0); }
}
```

Percentages let you choreograph several stages in one animation — impossible with a simple transition.

## The full animation shorthand

```css
.spinner {
  animation: spin 1s linear infinite;
  /*         │    │    │      └── iteration-count (infinite = loop forever) */
  /*         │    │    └───────── timing function */
  /*         │    └────────────── duration */
  /*         └─────────────────── @keyframes name */
}
@keyframes spin { to { transform: rotate(360deg); } } /* classic loading spinner */
```

Other properties: `animation-delay`, `animation-direction: alternate` (ping-pong), `animation-fill-mode: forwards` (keep the final state), `animation-play-state: paused`.

## Performance — animate transform/opacity only

```css
/* ✅ GPU-composited, 60fps */
@keyframes good { to { transform: scale(1.1); opacity: 0.5; } }
/* ❌ animating width/height/top forces layout every frame → jank */
@keyframes bad  { to { width: 300px; top: 50px; } }
```

Keyframe animations run on every frame, so animating layout-triggering properties is far more costly than animating `transform`/`opacity`.

## Accessibility

```css
@media (prefers-reduced-motion: reduce) {
  .spinner { animation: none; } /* honor users sensitive to motion */
}
```

## Why it matters

Keyframe animations power loaders, attention cues, entrances, and looping effects that transitions can't express (multi-step, auto-running, infinite).

Knowing the shorthand, sticking to `transform`/`opacity` for smoothness, and respecting `prefers-reduced-motion` lets you build rich motion that stays performant and accessible.