How do CSS transitions work?

Question

Accepted Answer

A **transition** smoothly animates a property from its old value to a new one over time, instead of changing it instantly. You declare *what* to animate and *how long*, then trigger it by changing the property (often via `:hover` or a class).

```css
.button {
  background: blue;
  transition: background 0.3s ease;  /* animate background over 300ms */
}
.button:hover {
  background: darkblue;  /* on hover, it fades smoothly instead of snapping */
}
```

## The four parts of the shorthand

```css
transition: background 0.3s ease 0s;
/*          │          │    │    └── delay before starting */
/*          │          │    └─────── timing function (the easing curve) */
/*          │          └──────────── duration */
/*          └─────────────────────── property to animate (or `all`) */
```

## Animating multiple properties

```css
.card {
  transition: transform 0.3s ease, box-shadow 0.3s ease;
}
.card:hover {
  transform: translateY(-4px);          /* lifts up */
  box-shadow: 0 8px 20px rgba(0,0,0,0.2); /* + deeper shadow */
}
```

List each property (comma-separated) rather than using `all`, which can accidentally animate things you didn't intend and hurt performance.

## Timing functions

```css
transition-timing-function: ease;        /* slow-fast-slow (default) */
/* linear | ease-in | ease-out | ease-in-out */
transition-timing-function: cubic-bezier(0.4, 0, 0.2, 1); /* custom curve */
```

## Performance: animate transform/opacity

```css
/* ✅ cheap — GPU-composited, no layout recalculation */
transition: transform 0.3s, opacity 0.3s;
/* ❌ expensive — animating width/top/left triggers layout on every frame */
transition: width 0.3s, left 0.3s;
```

Prefer animating `transform` and `opacity`; they're handled by the compositor without re-running layout, so they stay smooth (60fps).

## Accessibility

```css
@media (prefers-reduced-motion: reduce) {
  * { transition: none; } /* respect users who get motion sickness */
}
```

## Why it matters

Transitions add the polish that makes UI feel responsive and alive (hover effects, smooth state changes) with just one line.

Knowing to animate `transform`/`opacity` for performance — and to honor `prefers-reduced-motion` — is what separates smooth, accessible motion from janky animation.