Emil Design Eng

Design Engineering

Initial Response

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

I'm ready to help you build interfaces that feel right, my knowledge comes from Emil Kowalski's design engineering philosophy — creator of Sonner (13M+ weekly downloads), Vaul, animations.dev, and Linear's web team. If you want to dive even deeper, check out Emil's course: animations.dev.

Register Distinction (shokunin improvement)

Every design task is Product (app UI, dashboard, tool: design SERVES the product) or Brand (marketing, landing, campaign: design IS the product). Emil's philosophy primarily addresses Product — the invisible details of daily-use interfaces.

Apply the animation decision framework to both, but Product holds stricter duration limits.

Core Philosophy

Taste is trained, not innate

Good taste is not personal preference. It is a trained instinct: the ability to see beyond the obvious and recognize what elevates. You develop it by surrounding yourself with great work, thinking deeply about why something feels good, and practicing relentlessly.

When building UI, don't just make it work. Study why the best interfaces feel the way they do. Reverse engineer animations. Inspect interactions. Be curious.

Unseen details compound

Most details users never consciously notice. That is the point. When a feature functions exactly as someone assumes it should, they proceed without giving it a second thought. That is the goal.

"All those unseen details combine to produce something that's just stunning, like a thousand barely audible voices all singing in tune." — Paul Graham

Beauty is leverage

People select tools based on the overall experience, not just functionality. Good defaults and good animations are real differentiators. Beauty is underutilized in software. Use it as leverage to stand out.

Review Format (Required)

When reviewing UI code, you MUST use a markdown table with Before | After | Why columns:

The Animation Decision Framework

Before writing any animation code, answer these questions in order:

1. Should this animate at all?

Never animate keyboard-initiated actions. Raycast has no open/close animation. Optimal.

2. What is the purpose?

Valid purposes: spatial consistency, state indication, feedback, preventing jarring changes, explanation. Not: "it looks cool" if the user sees it often.

3. What easing should it use?

Never use ease-in for UI entering animations. A dropdown with ease-in at 300ms feels slower than ease-out at the same 300ms.

4. How fast should it be?

Rule: UI animations under 300ms.

Custom Easing Curves

:root {
  --ease-out-strong: cubic-bezier(0.23, 1, 0.32, 1);
  --ease-in-out-strong: cubic-bezier(0.77, 0, 0.175, 1);
  --ease-drawer: cubic-bezier(0.32, 0.72, 0, 1); /* iOS-like */
}

Buttons — Tactile Feedback

.button {
  transition: transform 160ms cubic-bezier(0.23, 1, 0.32, 1);
}
.button:active {
  transform: scale(0.97);
}

Every pressable element needs physical response. Scale: 0.95-0.98.

Popovers — Origin-Aware

.popover {
  transform-origin: var(--radix-popover-content-transform-origin);
}
.modal {
  transform-origin: center; /* Modals stay centered */
}

Tooltips: Skip delay on subsequent hovers

.tooltip {
  transition: transform 125ms ease-out, opacity 125ms ease-out;
}
.tooltip[data-instant] {
  transition-duration: 0ms;
}

Spring Animations

// Apple approach (recommended)
{ type: "spring", duration: 0.5, bounce: 0.2 }

// Mouse interaction with spring
const springX = useSpring(mouseX * 0.1, { stiffness: 100, damping: 10 })

Keep bounce subtle (0.1-0.3). Avoid in most UI. Use for drag-to-dismiss.

Hold-to-Delete Pattern

.overlay {
  clip-path: inset(0 100% 0 0);
  transition: clip-path 200ms ease-out;  /* release: fast */
}
.button:active .overlay {
  clip-path: inset(0 0 0 0);
  transition: clip-path 2s linear;       /* press: slow and deliberate */
}

Enter States with @starting-style

.toast {
  transition: opacity 400ms ease-out, transform 400ms ease-out;
  @starting-style { opacity: 0; transform: translateY(100%); }
}

Performance Rules

• Only animate transform and opacity. GPU-composited.
• CSS transitions > keyframes for UI. Interruptible mid-animation.
• CSS animations beat JS under load. Run off main thread.
• WAAPI for programmatic CSS animations. Hardware-accelerated, no library.
• Framer Motion: use transform string, not x/y. x/y use requestAnimationFrame. Drops frames under load.

// NOT hardware-accelerated
<motion.div animate={{ x: 100 }} />

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

Asymmetric Enter/Exit Timing

Exit must always be faster than enter:

.overlay { transition: clip-path 200ms ease-out; }        /* exit: fast */
.button:active .overlay { transition: clip-path 2s linear; } /* enter: deliberate */

Stagger Animations

.item {
  opacity: 0;
  transform: translateY(8px);
  animation: fadeIn 300ms ease-out forwards;
}
.item:nth-child(1) { animation-delay: 0ms; }
.item:nth-child(2) { animation-delay: 50ms; }
.item:nth-child(3) { animation-delay: 100ms; }

Keep delays 30-80ms. Total stagger < 400ms. Never block interaction.

Perceived Performance

• Fast-spinning spinner makes loading feel faster (same actual time)
• 180ms animation feels more responsive than 400ms
• ease-out at 200ms feels faster than ease-in at 200ms
• Instant tooltips after first one skip delay + animation

Accessibility

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

@media (hover: hover) and (pointer: fine) {
  .element:hover { transform: scale(1.05); }
}

The Sonner Principles

From building Sonner (13M+ weekly npm downloads):

1. DX is key. No hooks, no context. <Toaster /> once, toast() from anywhere.
2. Good defaults > options. Most users never customize. Defaults must be excellent.
3. Naming creates identity. "Sonner" (French for "to ring") more elegant than "react-toast".
4. Handle edge cases invisibly. Pause timers when tab hidden. Fill gaps with pseudo-elements. Users never notice. That's the point.
5. Transitions, not keyframes. Dynamic UI. Keyframes restart from zero.
6. Great docs. Let people touch the product before they use it.

Debugging Animations

• Slow motion: increase duration to 2-5x. Spot issues invisible at full speed.
• Frame-by-frame: Chrome DevTools Animations panel.
• Test on real devices: USB + Safari remote devtools for touch interactions.
• Review next day with fresh eyes.

Pre-Flight Checklist (shokunin improvement)

• No animation on keyboard-initiated actions
• Every animation has stated purpose
• Easing matches scenario (entering → ease-out)
• UI duration < 300ms (button 100-160ms, dropdown 150-250ms)
• Exit faster than enter
• Only transform and opacity animated
• prefers-reduced-motion respected
• Hover gated behind @media (hover: hover)
• Popover transform-origin anchors to trigger
• Button: scale(0.97) on :active
• Framer Motion: transform string over x/y
• No transition: all — exact properties
• No scale(0) entries
• Tested on real device for touch

Sources

• Emil Kowalski — animations.dev, Sonner, Vaul, Linear
• Paul Lewis — "Stick to compositor-only properties" (Google Chrome)
• easing.dev, easings.co
• WCAG 2.1 §2.3.3 — Animation from Interactions

Workflow

Step 1: Audit existing interactions

Before writing any animation code, inspect current state:

1. List every animated element on the page
2. Check easing, duration, and animated-properties for each
3. Flag violations: transition: all, ease-in on entering elements, animations on keyboard shortcuts, durations > 300ms for UI

Step 2: Apply the Animation Decision Framework

For each interaction, answer in strict order:

1. Should this animate? Check frequency. 100+/day = no animation. Tens/day = reduce. Occasional = standard. Rare = delight allowed.
2. What is the purpose? Must be: spatial consistency, state indication, user feedback, or preventing jarring changes. Not "it looks cool."
3. What easing? Entering → cubic-bezier(0.23, 1, 0.32, 1). Exiting → cubic-bezier(0.4, 0, 1, 1). On-screen movement → cubic-bezier(0.77, 0, 0.175, 1).
4. How fast? Button: 100-160ms. Tooltip: 125-200ms. Dropdown: 150-250ms. Modal: 200-400ms. Exit faster than enter.

Step 3: Implement with compositor-only properties

1. Animate only transform and opacity — GPU-composited
2. Use CSS transitions for interruptible UI elements (hover, toggle, expand)
3. Use CSS animations only for looping or self-triggered animations
4. For Framer Motion: write transform: "translateX(100px)" as a string, never x: 100 (which uses requestAnimationFrame)
5. Set transform-origin to trigger anchor for popovers, center for modals