agents-inc/web-animation-css-animations

CSS Animation Patterns

Quick Guide: Use CSS transitions for state changes (hover, focus), @keyframes for autonomous/looping animations, scroll-driven animations for scroll-linked effects. Animate only transform and opacity for 60fps. Always respect prefers-reduced-motion.

---

<critical_requirements>

CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST animate ONLY transform and opacity for GPU-accelerated 60fps performance)

(You MUST respect prefers-reduced-motion using @media (prefers-reduced-motion: no-preference) for opt-in or @media (prefers-reduced-motion: reduce) for opt-out)

(You MUST use CSS custom properties for ALL timing values - NO magic numbers like 0.3s)

(You MUST use ease-out for enter animations and ease-in for exit animations - NEVER linear for UI transitions)

(You MUST remove will-change after animation completes - permanent will-change wastes GPU memory)

</critical_requirements>

---

Auto-detection: CSS animation, CSS transition, @keyframes, transform, opacity, transition-duration, animation-duration, prefers-reduced-motion, scroll-timeline, animation-timeline, will-change, cubic-bezier, ease-out, ease-in, @property

When to use:

• Simple state change animations (hover, focus, active states)
• Autonomous looping animations (spinners, pulses, attention grabbers)
• Scroll-linked animations and parallax effects
• Micro-interactions that don't need JavaScript control

When NOT to use:

• Animations requiring JavaScript control (pause, reverse, seek) -- use Web Animations API
• Complex orchestrated animations with staggered timing -- use your animation library
• Physics-based spring animations -- use your animation library
• Drag-and-drop or gesture-driven animations -- use your animation library

Detailed Resources:

• examples/core.md - Token system, interactive states, shadows, loading, reduced motion
• examples/transitions.md - Multi-property transitions, accordions, color, links
• examples/keyframes.md - Scroll-driven, @property gradients, typewriter, stagger, shapes
• reference.md - Decision frameworks, timing reference, browser support

---

<philosophy>

Philosophy

CSS animations leverage the browser's compositor thread for smooth, 60fps animations that don't block JavaScript execution. By animating only GPU-accelerated properties (transform and opacity), animations run on a separate thread from the main JavaScript thread.

Core principles:

1. Performance first - Animate only transform and opacity to avoid layout/paint triggers
2. Accessibility built-in - Always respect prefers-reduced-motion user preferences
3. Transitions for state changes - Use CSS transitions for hover, focus, and state-driven animations
4. Keyframes for autonomous motion - Use @keyframes for animations that loop, auto-play, or have multiple steps
5. Design tokens for consistency - Use CSS custom properties for durations, easings, and distances

</philosophy>

---

<patterns>

Core Patterns

Pattern 1: Animation Token System

Define timing, easing, and distance tokens as CSS custom properties for consistency. See examples/core.md for the full token setup.

:root {
  --duration-instant: 100ms;
  --duration-fast: 150ms;
  --duration-normal: 250ms;
  --duration-slow: 400ms;

  --ease-out: cubic-bezier(0, 0, 0.2, 1); /* Enter */
  --ease-in: cubic-bezier(0.4, 0, 1, 1); /* Exit */
  --ease-in-out: cubic-bezier(0.4, 0, 0.2, 1); /* Symmetric */
  --ease-spring: cubic-bezier(0.175, 0.885, 0.32, 1.275); /* Bouncy */

  --lift-sm: -2px;
  --lift-md: -4px;
}

Why tokens matter: Consistent timing across application, easy to adjust globally, semantic naming communicates intent

---

Pattern 2: GPU-Accelerated Transitions

Only animate transform and opacity. Never animate layout properties like width, height, top, left, margin, or padding.

/* CORRECT - GPU-accelerated */
.card {
  transition:
    transform var(--duration-fast) var(--ease-out),
    opacity var(--duration-fast) var(--ease-out);
}
.card:hover {
  transform: translateY(var(--lift-md)) scale(1.02);
}

/* WRONG - triggers layout recalculation every frame */
.card {
  transition: all 0.3s linear;
}
.card:hover {
  top: -8px;
  margin-top: -8px;
}

Transform mapping: Use translate() instead of top/left, scale() instead of width/height, pseudo-element opacity instead of box-shadow.

See examples/core.md for button states, card hover effects, and the pseudo-element shadow technique.

---

Pattern 3: Prefers-Reduced-Motion

Every animation must respect user motion preferences. Two strategies:

Progressive Enhancement (Recommended)

/* Base: no motion */
.element {
  opacity: 1;
  transform: translateY(0);
}

/* Opt-in to motion */
@media (prefers-reduced-motion: no-preference) {
  .element {
    animation: fade-slide-in var(--duration-normal) var(--ease-out);
  }
}

Graceful Degradation

.notification {
  animation: slide-in-bounce var(--notification-duration) var(--ease-spring);
}

@media (prefers-reduced-motion: reduce) {
  .notification {
    animation: fade-in calc(var(--notification-duration) * 0.5) var(--ease-out);
  }
}

Key insight: Reduced motion does not mean no animation. Opacity fades are generally safe. Replace spatial movement with opacity-only alternatives.

See examples/core.md for the complete reduced motion pattern.

---

Pattern 4: CSS @keyframes

Use @keyframes for animations that loop, auto-play on mount, or have more than two states.

@keyframes fade-slide-in {
  from {
    opacity: 0;
    transform: translateY(20px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

.modal {
  --modal-enter-duration: 300ms;
  animation: fade-slide-in var(--modal-enter-duration) var(--ease-out) forwards;
}

Key details:

• Use forwards fill mode to retain final state after animation
• Use backwards fill mode to show initial state during animation-delay
• Use ease-out for enter, ease-in for exit
• linear is only appropriate for continuous rotation (spinners)

See examples/core.md for spinners, pulses, skeleton loaders, and toast animations. See examples/keyframes.md for scroll-driven animations, @property gradients, and complex sequences.

---

Pattern 5: Will-Change Optimization

will-change creates a GPU layer (~307KB per 320x240px element). Apply only when needed, remove after.

/* CORRECT - only during interaction */
.card:hover {
  will-change: transform;
}

/* WRONG - permanent GPU layer on every element */
* {
  will-change: transform;
}

Never apply will-change permanently. Each element with will-change creates a separate compositing layer that consumes GPU memory. On mobile devices, this can crash the browser.

---

Pattern 6: Scroll-Driven Animations

CSS animation-timeline allows scroll-linked animations without JavaScript.

.progress-bar {
  animation: grow-width linear;
  animation-timeline: scroll();
}

@keyframes grow-width {
  from {
    transform: scaleX(0);
  }
  to {
    transform: scaleX(1);
  }
}

Two timeline types:

• scroll() -- progress based on scroll container position
• view() -- progress based on element visibility in viewport

Browser support: Chrome/Edge 115+, Safari 26+, Firefox behind flag

See examples/keyframes.md for scroll progress, viewport reveal, and parallax patterns.

---

Pattern 7: @property for Custom Property Animation

CSS Houdini's @property enables animating custom properties like gradient angles that CSS cannot normally interpolate.

@property --gradient-angle {
  syntax: "<angle>";
  initial-value: 0deg;
  inherits: false;
}

.gradient-border {
  background: linear-gradient(var(--gradient-angle), #ff0080, #7928ca);
  animation: rotate-gradient 3s linear infinite;
}

@keyframes rotate-gradient {
  to {
    --gradient-angle: 360deg;
  }
}

Browser support: Chrome/Edge 85+, Safari 16.4+, Firefox 128+

</patterns>

---

<performance>

Performance

The 16.67ms Budget

For 60fps, each frame must complete in 16.67ms. Layout-triggering animations often exceed this budget.

| Category | Properties | Impact | | -------------------------- | ----------------------------------------- | ------------------------------------ | | Composite only (Best) | transform, opacity | No layout, no paint, GPU-accelerated | | Paint only (Okay) | color, background-color, visibility | No layout, but repaints | | Layout + Paint (Avoid) | width, height, margin, padding, top, left | Full page recalculation |

Duration Guidelines

| Animation Type | Duration | Reason | | ------------------ | ---------- | ------------------------- | | Micro-interactions | 100-150ms | Feels instant | | UI transitions | 200-300ms | Sweet spot for perception | | Page transitions | 300-500ms | Major context change | | Complex sequences | 500-1000ms | Story-telling moments |

Transform Mapping

| Instead of... | Use... | | ------------------- | ------------------------------- | | top, left | translate(x, y) | | width, height | scale() | | box-shadow | Pseudo-element with opacity | | margin, padding | translate() with layout space |

</performance>

---

<decision_framework>

Decision Framework

Transitions vs @keyframes

Is the animation triggered by user interaction (hover, focus, class toggle)?
├─ YES → Is it a simple A->B state change?
│   ├─ YES -> CSS Transition
│   └─ NO -> Does it need multiple steps?
│       ├─ YES -> CSS @keyframes
│       └─ NO -> CSS Transition is fine
└─ NO -> Does it auto-play or loop?
    ├─ YES -> CSS @keyframes
    └─ NO -> CSS Transition (triggered by class toggle)

Easing Selection

What type of motion?
├─ Element entering -> ease-out (fast start, slow end)
├─ Element exiting -> ease-in (slow start, fast end)
├─ Symmetric motion -> ease-in-out
├─ Continuous rotation -> linear
├─ Playful/bouncy -> custom cubic-bezier with overshoot
└─ Default UI -> ease-out

Never use:
├─ linear for UI transitions (feels robotic)
└─ ease (browser default) for production (too generic)

CSS vs JavaScript Animation

Does the animation need...
├─ Pause/play/reverse/seek control? -> JavaScript (Web Animations API)
├─ Dynamic values calculated at runtime? -> JavaScript or CSS custom properties
├─ Physics-based springs? -> Your animation library
├─ Orchestrated staggering across many elements? -> JavaScript for complex, CSS for simple
├─ Scroll-linked progress? -> CSS scroll-driven animations
├─ Page/view transitions? -> See the View Transitions skill
└─ Simple state transitions? -> CSS Transitions

</decision_framework>

---

<red_flags>

RED FLAGS

High Priority

• Animating layout properties (width, height, top, left, margin, padding) -- triggers expensive reflows every frame; use transform instead
• Magic numbers for timing (0.3s, 300ms inline) -- all durations must be CSS custom properties
• Missing prefers-reduced-motion -- every animation must respect user preferences
• Linear easing for UI transitions -- linear feels robotic; use ease-out for enter, ease-in for exit
• Permanent will-change -- creates GPU layers permanently, wasting memory; apply only during animation

Medium Priority

• Using transition: all -- transitions unnecessary properties, causes surprises when new properties are added
• Animating box-shadow directly -- causes repaint every frame; use pseudo-element with opacity
• Missing forwards on enter animations -- element snaps back to initial state
• Very long durations (>1s) -- users perceive as slow; rarely appropriate outside special effects

Gotchas & Edge Cases

• transform + position: fixed -- transform creates new containing block, breaking fixed positioning relative to viewport
• will-change creates stacking context -- can affect z-index behavior unexpectedly
• Cannot animate display: none -- use opacity + visibility or grid-template-rows: 0fr
• fill-mode: backwards needed for delayed animations -- without it, element shows in final state during delay
• SVG uses different properties -- animate stroke-dashoffset and stroke-dasharray, not transform for path drawing
• Scroll-driven animations need scrollable container -- overflow: hidden parent breaks scroll-timeline
• Print media -- animations don't print; ensure content is visible without animation

</red_flags>

---

<critical_reminders>

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md

(You MUST animate ONLY transform and opacity for GPU-accelerated 60fps performance)

(You MUST respect prefers-reduced-motion using @media (prefers-reduced-motion: no-preference) for opt-in or @media (prefers-reduced-motion: reduce) for opt-out)

(You MUST use CSS custom properties for ALL timing values - NO magic numbers like 0.3s)

(You MUST use ease-out for enter animations and ease-in for exit animations - NEVER linear for UI transitions)

(You MUST remove will-change after animation completes - permanent will-change wastes GPU memory)

Failure to follow these rules will cause jank, accessibility issues, and degraded user experience.

</critical_reminders>