animations

GPU-Accelerated Properties

Only these properties animate on the compositor thread (60fps):

| Property | Use | |----------|-----| | transform | Move, rotate, scale (translateX, rotate, scale) | | opacity | Fade in/out |

Everything else triggers layout or paint. Avoid animating:

• width, height, margin, padding (layout thrashing)
• top, left, right, bottom (use transform instead)
• border-width, font-size (expensive reflows)

/* ❌ Triggers layout every frame */
.slide { left: 100px; transition: left 0.3s; }

/* ✅ GPU accelerated */
.slide { transform: translateX(100px); transition: transform 0.3s; }

Reduced Motion

~5% of users experience vestibular disorders (dizziness, nausea from motion).

/* Only animate if user hasn't requested reduced motion */
@media (prefers-reduced-motion: no-preference) {
  .animated { animation: slide-in 0.5s ease-out; }
}

/* Or disable for those who requested it */
@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
  }
}

Keep subtle fades and color changes even with reduced motion. Remove parallax, bouncing, infinite loops.

Timing Functions

| Easing | Use case | |--------|----------| | ease-out | Elements entering view (appears responsive) | | ease-in | Elements exiting view (accelerates away) | | ease-in-out | Elements moving within view | | linear | Only for spinners, progress bars, color cycling |

/* Custom bounce */
transition: transform 0.4s cubic-bezier(0.68, -0.55, 0.265, 1.55);

/* Material Design standard */
transition: transform 0.3s cubic-bezier(0.4, 0, 0.2, 1);

Duration Guidelines

| Type | Duration | |------|----------| | Micro-interactions (hover, focus) | 100-200ms | | Transitions (modals, dropdowns) | 200-300ms | | Page transitions | 300-500ms | | Complex choreography | 500-1000ms max |

Anything over 500ms feels sluggish. Shorter is usually better.

CSS Transitions vs Animations

Transitions: A to B state changes

.button { transform: scale(1); transition: transform 0.2s ease-out; }
.button:hover { transform: scale(1.05); }

Animations: Multi-step, auto-play, looping

@keyframes fadeSlideIn {
  0% { opacity: 0; transform: translateY(20px); }
  100% { opacity: 1; transform: translateY(0); }
}
.card { animation: fadeSlideIn 0.5s ease-out forwards; }

Use transitions for hover/focus states. Use animations for on-load effects and sequences.

will-change

Only use as optimization when you have specific performance problems:

/* ✅ Apply before animation starts, remove after */
.card:hover { will-change: transform; }
.card { will-change: auto; }

/* ❌ Never apply globally */
* { will-change: transform, opacity; }  /* Wastes GPU memory */

Transition Property Specificity

/* ❌ Animates everything including unintended properties */
.card { transition: all 0.3s; }

/* ✅ Only animates what you need */
.card { transition: transform 0.3s, box-shadow 0.3s; }

all can cause unexpected animations on color, background, border changes.

React/Framework Traps

Exit animations require AnimatePresence:

/* ❌ Exit animation never plays */
{isVisible && <motion.div exit={{ opacity: 0 }} />}

/* ✅ Wrap conditional rendering */
<AnimatePresence>
  {isVisible && <motion.div exit={{ opacity: 0 }} />}
</AnimatePresence>

Stable keys for list animations:

/* ❌ Index keys cause erratic animations */
{items.map((item, i) => <li key={i}>{item}</li>)}

/* ✅ Stable IDs */
{items.map(item => <li key={item.id}>{item.text}</li>)}

AutoAnimate parent must be unconditional:

/* ❌ Ref can't attach to conditional element */
{showList && <ul ref={parent}>...</ul>}

/* ✅ Parent always renders, children are conditional */
<ul ref={parent}>{showList && items.map(...)}</ul>

Library Selection

| Library | Size | Best for | |---------|------|----------| | CSS only | 0kb | Hover states, simple transitions | | AutoAnimate | 3kb | Lists, accordions, toasts (90% of UI animations) | | Motion | 22kb | Gestures, physics, scroll animations, complex choreography | | GSAP | 60kb | Timelines, creative animation, scroll-triggered sequences |

Start with CSS. Add AutoAnimate for list animations. Only add Motion/GSAP for complex needs.

Common Mistakes

• Animating width/height instead of scale—causes layout thrashing
• Infinite animations without pause control—no way to stop
• Same easing for enter and exit—use ease-out for enter, ease-in for exit
• Ignoring prefers-reduced-motion—causes physical discomfort
• Duration over 500ms—feels sluggish
• transition: all catching unintended properties
• Missing AnimatePresence for exit animations in React