web-animation-design（网页动画设计）

Web Animation Design

A comprehensive guide for creating animations that feel right, based on Emil Kowalski's "Animations on the Web" course.

Initial Response

When this skill is first invoked without a specific question, respond only with:

I'm ready to help you with animations based on Emil Kowalski's animations.dev course.

Do not provide any other information until the user asks a question.

Review Format (Required)

When reviewing animations, you MUST use a markdown table. Do NOT use a list with "Before:" and "After:" on separate lines. Always output an actual markdown table like this:

Wrong format (never do this):

Before: transform: scale(0)
After: transform: scale(0.95)
────────────────────────────
Before: 400ms duration
After: 200ms

Correct format: A single markdown table with | Before | After | columns, one row per issue.

Quick Start

Every animation decision starts with these questions:

1. Is this element entering or exiting? → Use ease-out
2. Is an on-screen element moving? → Use ease-in-out
3. Is this a hover/color transition? → Use ease
4. Will users see this 100+ times daily? → Don't animate it

The Easing Blueprint

ease-out (Most Common)

Use for user-initiated interactions: dropdowns, modals, tooltips, any element entering or exiting the screen.

/* Sorted weak to strong */
--ease-out-quad: cubic-bezier(0.25, 0.46, 0.45, 0.94);
--ease-out-cubic: cubic-bezier(0.215, 0.61, 0.355, 1);
--ease-out-quart: cubic-bezier(0.165, 0.84, 0.44, 1);
--ease-out-quint: cubic-bezier(0.23, 1, 0.32, 1);
--ease-out-expo: cubic-bezier(0.19, 1, 0.22, 1);
--ease-out-circ: cubic-bezier(0.075, 0.82, 0.165, 1);

Why it works: Acceleration at the start creates an instant, responsive feeling. The element "jumps" toward its destination then settles in.

ease-in-out (For Movement)

Use when elements already on screen need to move or morph. Mimics natural motion like a car accelerating then braking.

/* Sorted weak to strong */
--ease-in-out-quad: cubic-bezier(0.455, 0.03, 0.515, 0.955);
--ease-in-out-cubic: cubic-bezier(0.645, 0.045, 0.355, 1);
--ease-in-out-quart: cubic-bezier(0.77, 0, 0.175, 1);
--ease-in-out-quint: cubic-bezier(0.86, 0, 0.07, 1);
--ease-in-out-expo: cubic-bezier(1, 0, 0, 1);
--ease-in-out-circ: cubic-bezier(0.785, 0.135, 0.15, 0.86);

ease (For Hover Effects)

Use for hover states and color transitions. The asymmetrical curve (faster start, slower end) feels elegant for gentle animations.

transition: background-color 150ms ease;

linear (Avoid in UI)

Only use for:

• Constant-speed animations (marquees, tickers)
• Time visualization (hold-to-delete progress indicators)

Linear feels robotic and unnatural for interactive elements.

ease-in (Almost Never)

Avoid for UI animations. Makes interfaces feel sluggish because the slow start delays visual feedback.

Paired Elements Rule

Elements that animate together must use the same easing and duration. Modal + overlay, tooltip + arrow, drawer + backdrop—if they move as a unit, they should feel like a unit.

/* Both use the same timing */
.modal { transition: transform 200ms ease-out; }
.overlay { transition: opacity 200ms ease-out; }

Timing and Duration

Duration Guidelines

Rule: UI animations should stay under 300ms. Larger elements animate slower than smaller ones.

The Frequency Principle

Determine how often users will see the animation:

• 100+ times/day → No animation (or drastically reduced)
• Occasional use → Standard animation
• Rare/first-time → Can add delight

Example: Raycast never animates its menu toggle because users open it hundreds of times daily.

When to Animate

Do animate:

• Enter/exit transitions for spatial consistency
• State changes that benefit from visual continuity
• Responses to user actions (feedback)
• Rarely-used interactions where delight adds value

Don't animate:

• Keyboard-initiated actions
• Hover effects on frequently-used elements
• Anything users interact with 100+ times daily
• When speed matters more than smoothness

Marketing vs. Product:

• Marketing: More elaborate, longer durations allowed
• Product: Fast, purposeful, never frivolous

Spring Animations

Springs feel more natural because they don't have fixed durations—they simulate real physics.

When to Use Springs

• Drag interactions with momentum
• Elements that should feel "alive" (Dynamic Island)
• Gestures that can be interrupted mid-animation
• Organic, playful interfaces

Configuration

Apple's approach (recommended):

// Duration + bounce (easier to understand)
{ type: "spring", duration: 0.5, bounce: 0.2 }

Traditional physics:

// Mass, stiffness, damping (more complex)
{ type: "spring", mass: 1, stiffness: 100, damping: 10 }

Bounce Guidelines

• Avoid bounce in most UI contexts
• Use bounce for drag-to-dismiss, playful interactions
• Keep bounce subtle (0.1-0.3) when used

Interruptibility

Springs maintain velocity when interrupted—CSS animations restart from zero. This makes springs ideal for gestures users might change mid-motion.

Performance

The Golden Rule

Only animate transform and opacity. These skip layout and paint stages, running entirely on the GPU.

Avoid animating:

• padding, margin, height, width (trigger layout)
• blur filters above 20px (expensive, especially Safari)
• CSS variables in deep component trees

Optimization Techniques

/* Force GPU acceleration */
.animated-element {
  will-change: transform;
}

React-specific:

• Animate outside React's render cycle when possible
• Use refs to update styles directly instead of state
• Re-renders on every frame = dropped frames

Framer Motion:

// Hardware accelerated (transform as string)
<motion.div animate={{ transform: "translateX(100px)" }} />

// NOT hardware accelerated (more readable)
<motion.div animate={{ x: 100 }} />

CSS vs. JavaScript

• CSS animations run off main thread (smoother under load)
• JS animations (Framer Motion, React Spring) use requestAnimationFrame
• CSS better for simple, predetermined animations
• JS better for dynamic, interruptible animations

Accessibility

Animations can cause motion sickness or distraction for some users.

prefers-reduced-motion

Whenever you add an animation, also add a media query to disable it:

.modal {
  animation: fadeIn 200ms ease-out;
}

@media (prefers-reduced-motion: reduce) {
  .modal {
    animation: none;
  }
}

Reduced Motion Guidelines

• Every animated element needs its own prefers-reduced-motion media query
• Set animation: none or transition: none (no !important)
• No exceptions for opacity or color - disable all animations
• Show play buttons instead of autoplay videos

Framer Motion Implementation

import { useReducedMotion } from "framer-motion";

function Component() {
  const shouldReduceMotion = useReducedMotion();

  return (
    <motion.div
      initial={shouldReduceMotion ? false : { opacity: 0, y: 20 }}
      animate={{ opacity: 1, y: 0 }}
    />
  );
}

Touch Device Considerations

/* Disable hover animations on touch devices */
@media (hover: hover) and (pointer: fine) {
  .element:hover {
    transform: scale(1.05);
  }
}

Touch devices trigger hover on tap, causing false positives.

Practical Tips

Quick reference for common scenarios. See PRACTICAL-TIPS.md for detailed implementations.

Easing Decision Flowchart

Is the element entering or exiting the viewport? ├── Yes → ease-out └── No ├── Is it moving/morphing on screen? │ └── Yes → ease-in-out └── Is it a hover change? ├── Yes → ease └── Is it constant motion? ├── Yes → linear └── Default → ease-out

Reference Files

• PRACTICAL-TIPS.md - Detailed implementations for common animation scenarios