Css Animation Performance: Performance-Optimized Motion for Modern UIs

// Four stages: JavaScript → Style → Layout → Paint → Composite
//
// Composite only (GPU, 60fps guaranteed):
// - transform: translateX, translateY, scaleX, scaleY, rotate, skew
// - opacity
//
// Triggers Paint (slower, but ok for occasional changes):
// - color, background-color, border-color, box-shadow
//
// Triggers Layout (SLOW — recalculates entire page):
// - width, height, padding, margin, top, left, right, bottom
// - font-size, line-height, display, float, position
//
// Rule: animate ONLY transform and opacity for smooth 60fps

/* WRONG: animating position properties causes layout thrashing */
@keyframes slideInBad {
  from { left: -100px; }  /* Triggers layout on every frame */
  to   { left: 0; }
}

/* RIGHT: animating transform — GPU compositor only */
@keyframes slideIn {
  from { transform: translateX(-100px); }  /* No layout, no paint, GPU only */
  to   { transform: translateX(0); }
}

.element {
  animation: slideIn 0.3s ease-out;
}

/* Transitions — same principle */
/* WRONG: */
.btn { transition: width 0.3s; }
.btn:hover { width: 200px; }  /* Layout on every frame */

/* RIGHT: */
.btn { transition: transform 0.3s; }
.btn:hover { transform: scaleX(1.1); }  /* GPU only */

/* will-change: tell browser to promote element to GPU layer in advance */
.animated-card {
  will-change: transform, opacity; /* GPU layer created before animation starts */
}

/* Apply only to elements you KNOW will animate */
/* NOT on every element — each GPU layer uses VRAM */
/* WRONG: */
* { will-change: transform; }  /* Crashes GPU with thousands of layers */

/* Pattern: apply on hover/focus, remove when done */
.card:hover {
  will-change: transform;
}
.card {
  transition: transform 0.3s;
}
.card:hover {
  transform: translateY(-8px);
}
.card:not(:hover) {
  will-change: auto; /* Release GPU layer when not animating */
}

/* Entrance animation — fade + slide */
@keyframes fadeInUp {
  from {
    opacity: 0;
    transform: translateY(20px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

.card {
  animation: fadeInUp 0.4s ease-out both;
}

/* Stagger: delay each child */
.card:nth-child(2) { animation-delay: 0.1s; }
.card:nth-child(3) { animation-delay: 0.2s; }

/* Respect prefers-reduced-motion */
@media (prefers-reduced-motion: reduce) {
  * {
    animation-duration: 0.01ms !important;
    transition-duration: 0.01ms !important;
  }
}