CSS Animations & Transitions: Everything You Need to Know

What it does: Smoothly animates property changes over time instead of instant jumps. Why use it: Creates polished, professional interfaces. Users perceive smooth transitions as higher quality. How it works:
1. Browser detects property change (hover, class addition, etc.)
2. Calculates intermediate values between start and end states
3. Updates property gradually over specified duration

.element {
  / Shorthand: property duration timing-function delay /
  transition: opacity 0.3s ease-in-out 0s;
/ Individual properties /
  transition-property: opacity;
  transition-duration: 0.3s;
  transition-timing-function: ease-in-out;
  transition-delay: 0s;
/ Multiple properties /
  transition: opacity 0.3s, transform 0.5s;
/ All properties /
  transition: all 0.3s ease;
}

/ Basic keyframes /
@keyframes fade-in {
  from {
    opacity: 0;
  }
  to {
    opacity: 1;
  }
}
/ Percentage-based keyframes /
@keyframes slide-bounce {
  0% {
    transform: translateX(0);
  }
  25% {
    transform: translateX(100px);
  }
  50% {
    transform: translateX(100px) translateY(-20px);
  }
  75% {
    transform: translateX(100px) translateY(0);
  }
  100% {
    transform: translateX(0);
  }
}

.spinner {
  width: 40px;
  height: 40px;
  border: 4px solid #f3f3f3;
  border-top: 4px solid #3498db;
  border-radius: 50%;
  animation: spin 1s linear infinite;
}
@keyframes spin {
  to { transform: rotate(360deg); }
}

What it does: Offloads animations to GPU, achieving 60fps even on complex animations. Why it matters: Janky animations frustrate users and make apps feel broken. GPU acceleration = smooth motion. How it works:

• GPU handles: transform, opacity, filter
• CPU handles: width, height, top, left, margin, padding
• GPU creates compositor layers for animated elements

/ ✅ GOOD - GPU accelerated properties /
.performant {
  transform: translateX(100px);  / Position via transform /
  opacity: 0.5;                  / Transparency /
  filter: blur(5px);             / Visual effects /
  / These run at 60fps without affecting other elements /
}
/ ❌ AVOID - Triggers expensive layout/paint /
.expensive {
  left: 100px;     / Forces layout recalculation /
  width: 200px;    / All elements must reflow /
  height: 100px;   / Entire page might repaint /
  / These cause "layout thrashing" - major performance hit /
}
/ Real example: Sliding panel /
/ ❌ Bad - animates 'left' property /
.panel-slow {
  position: absolute;
  left: -300px;
  transition: left 0.3s;
}
.panel-slow.open {
  left: 0;
}
/ ✅ Good - animates 'transform' /
.panel-fast {
  transform: translateX(-300px);
  transition: transform 0.3s;
}
.panel-fast.open {
  transform: translateX(0);
}

.flip-card {
  width: 300px;
  height: 200px;
  perspective: 1000px;
}
.flip-card-inner {
  position: relative;
  width: 100%;
  height: 100%;
  transition: transform 0.6s;
  transform-style: preserve-3d;
}
.flip-card:hover .flip-card-inner {
  transform: rotateY(180deg);
}
.flip-card-front, .flip-card-back {
  position: absolute;
  width: 100%;
  height: 100%;
  backface-visibility: hidden;
}
.flip-card-back {
  transform: rotateY(180deg);
}

.video-animation {
  animation: rotate 2s linear infinite;
  animation-play-state: paused;
}
.playing .video-animation {
  animation-play-state: running;
}

What it does: Detects if users have requested reduced motion in their OS settings. Why it matters: Motion can trigger vestibular disorders, migraines, and motion sickness. It's not just preference - it's accessibility. How to implement: Provide alternatives, not just removal. Replace motion with fades or instant changes.

/ Method 1: Nuclear option - remove all motion /
@media (prefers-reduced-motion: reduce) {
  *,
  *::before,
  *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
    scroll-behavior: auto !important;
  }
}
/ Method 2: Better - provide alternatives /
.card {
  animation: slideUp 0.5s ease;
}
@media (prefers-reduced-motion: reduce) {
  .card {
    animation: fadeIn 0.2s ease; / Gentler alternative /
  }
}
/ Method 3: Best - design with preference in mind /
.animated-element {
  / Default: subtle motion /
  transform: scale(1);
  transition: transform 0.2s ease;
}
.animated-element:hover {
  transform: scale(1.05); / Small, purposeful /
}
@media (prefers-reduced-motion: no-preference) {
  / Enhance for those who want motion /
  .animated-element {
    transition: transform 0.3s cubic-bezier(0.68, -0.55, 0.265, 1.55);
  }
  .animated-element:hover {
    transform: scale(1.1) rotate(5deg);
  }
}

• macOS: System Preferences > Accessibility > Display > Reduce motion
• Windows: Settings > Ease of Access > Display > Show animations
• Chrome DevTools: Rendering tab > Emulate CSS media feature prefers-reduced-motion