Building Animations

A construction skill. It does ONE thing: turn a request for motion into an implementation that would survive a strict review. It does not audit a codebase (that's improve-animations), critique a diff (that's review-animations), or hunt for places that could animate (that's find-animation-opportunities).

Operating Posture

You are a senior design engineer building the animation yourself. The bar is Emil Kowalski's animation philosophy — the same bar review-animations enforces. Write it so it passes that review the first time.

Two failure modes, and the first is worse:

1. Animating something that shouldn't animate. The gate below exists to produce zero lines of code sometimes. That's a success, not a dodge.
2. Animating the right thing with the wrong ingredients — ease-in on an entrance, scale(0), keyframes on a toast, a duration that makes a dropdown feel sluggish.

Never present motion options as a menu. Make the call, state the reasoning in one line, write the code.

Hard Rules

1. Run the sequence in order. Steps 1 and 2 gate everything. Don't reach for a curve before you know whether it animates at all.
2. No approximated values. Every curve, duration, and spring config comes from the tables below. Never invent cubic-bezier(0.4, 0, 0.2, 1) because it looks familiar.
3. Extend the codebase's tokens, don't fork them. If --ease-out or a duration scale already exists, use it. Adding a parallel system is a defect.
4. Reduced motion and hover gating ship with the animation, not as a follow-up.
5. Cheapest tool that works. Don't install a motion library for a fade.

The Build Sequence

1. Should this animate at all?

Frequency	Decision
100+ times/day (keyboard shortcuts, command palette toggle)	No animation. Ever. Stop here.
Tens of times/day (hover effects, list navigation)	Near-imperceptible only — fast and subtle, or nothing
Occasional (modals, drawers, toasts)	Standard animation
Rare / first-time (onboarding, success, celebration)	The delight budget lives here

Keyboard-initiated actions are a disqualifier, not a judgment call. Raycast has no open/close animation — that is correct for something opened hundreds of times a day.

If the request fails this gate, say so plainly and don't write the animation. Offer the non-motion alternative (instant state change, a static affordance) instead.

2. What is the purpose?

Name it in one of these words before continuing:

• Feedback — confirming the interface heard the user
• Spatial consistency — showing where something came from or went
• State indication — making a state change legible
• Preventing a jarring change — bridging content that would otherwise teleport
• Explanation — demonstrating how something works (marketing/onboarding only)
• Delight — allowed only at the rare/first-time tier

Can't name it? Don't build it. "It looks cool" on a frequently-seen element is a reason to stop.

Also check function: data the user is reading or acting on should not move for style. A decorative mouse-tracking effect belongs on a marketing page, not on a graph in a banking app.

3. Pick the tool — cheapest that works

Walk down; stop at the first that fits.

Need	Tool
Hover, press, color, a state toggle you control with a class or attribute	CSS transition
Entry animation on mount, no JS state	CSS @starting-style
Predetermined motion that must stay smooth while the page is busy loading	CSS animation (runs off the main thread)
Programmatic control with CSS performance, no library	WAAPI (element.animate())
Springs, layout animations, exit animations, gesture-driven values	Motion (motion.dev)

CSS animations beat JS under load — they run off the main thread, while requestAnimationFrame-based animation drops frames while the browser loads, scripts, or paints. Use CSS for predetermined motion, JS for dynamic and interruptible motion.

If the task needs a component rather than an animation — a toast, a drawer, a command menu, a dropdown — stop and invoke pick-ui-library. Hand-rolling those is how you end up with a <div> dropdown and no focus management.

4. Pick the properties

• transform and opacity only. They skip layout and paint and run on the GPU. width/height/margin/padding/top/left trigger all three. (clip-path is the sanctioned fourth — see RECIPES.md. height is tolerated only for accordions, where there's no transform equivalent.)
• Never scale(0). Start from scale(0.9–0.97) + opacity: 0. Nothing in the real world appears from nothing.
• transform-origin at the trigger for popovers, dropdowns, menus, tooltips — var(--transform-origin) in Base UI. Modals are exempt; they're not anchored to a trigger, so they stay centered.
• Percentages in translate() are relative to the element's own size — translateY(100%) moves by its own height whatever the content. Prefer over hardcoded pixels.
• In Motion, use the full transform string. x/y/scale shorthands are not hardware-accelerated and drop frames under load:

<motion.div animate={{ x: 100 }} />                          // drops frames under load
<motion.div animate={{ transform: "translateX(100px)" }} />  // hardware accelerated

• Never drive a child's transform from a CSS variable on the parent — it recalculates styles for every child. Set transform on the element directly.

5. Easing and duration — or a spring

Easing, in decision order:

Situation	Easing
Entering or exiting	ease-out
Moving / morphing on screen	ease-in-out
Hover / color change	ease
Constant motion (marquee, progress)	linear
Default	ease-out

Never ease-in on UI. It starts slow, delaying the exact moment the user is watching. ease-out at 200ms feels faster than ease-in at 200ms.

Built-in CSS easings are too weak. Use these:

--ease-out: cubic-bezier(0.23, 1, 0.32, 1);        /* strong ease-out for UI */
--ease-in-out: cubic-bezier(0.77, 0, 0.175, 1);    /* strong ease-in-out for on-screen movement */
--ease-drawer: cubic-bezier(0.32, 0.72, 0, 1);     /* iOS-like drawer curve (Ionic) */

Need a curve that isn't here? Take it from easing.dev or easings.co. Don't hand-roll one.

Duration:

Element	Duration
Button press feedback	100–160ms
Tooltips, small popovers	125–200ms
Dropdowns, selects	150–250ms
Modals, drawers	200–500ms
Marketing / explanatory	Can be longer

UI animations stay under 300ms. A 180ms dropdown feels more responsive than a 400ms one.

Reach for a spring instead when the motion is drag with momentum, an element that should feel alive, a gesture the user can interrupt or reverse, or decorative mouse-tracking:

{ type: "spring", duration: 0.5, bounce: 0.2 }        // Apple-style — easier to reason about
{ type: "spring", mass: 1, stiffness: 100, damping: 10 }  // traditional physics — more control

Keep bounce at 0.1–0.3, and avoid bounce in most UI — reserve it for drag-to-dismiss and playful interactions.

6. Interruption and exit

• Transitions, not keyframes, for anything triggered rapidly — toasts, toggles, anything a user can fire twice in a second. Transitions retarget from the current value; keyframes restart from zero.
• Springs for gestures, because they carry velocity through an interruption.
• Exit the way it entered. A toast that slides in from the bottom leaves through the bottom. Symmetric paths are what make swipe-to-dismiss feel obvious.
• Asymmetric timing where the user is deciding. Slow on the deliberate phase (a hold-to-confirm press: 2s linear), snappy on the system response (release: 200ms ease-out).

7. Reduced motion and pointer gating

Ships with the animation, every time.

@media (prefers-reduced-motion: reduce) {
  .element { animation: fade 0.2s ease; } /* keep opacity/color, drop transform-based motion */
}

@media (hover: hover) and (pointer: fine) {
  .element:hover { transform: scale(1.05); } /* touch fires false hovers on tap */
}

const reduce = useReducedMotion();
const closedX = reduce ? 0 : '-100%';

Reduced motion means fewer and gentler animations, not zero — keep transitions that aid comprehension, remove movement and position changes.

Recipes

For ready-to-build implementations of the common cases — button press, dropdown, tooltip, modal, drawer, toast, accordion, stagger, hold-to-confirm, tab indicator, scroll reveal, drag-to-dismiss — see RECIPES.md. Load it whenever the request matches one of those components; start from the recipe rather than from a blank file.

Never Ship

Self-check before you finish. Each of these is an automatic block in review-animations:

Never	Instead
transition: all	Name the exact properties
transform: scale(0) entrance	scale(0.95) + opacity: 0
ease-in on a UI element	ease-out or a strong custom curve
Built-in ease-out on a deliberate animation	cubic-bezier(0.23, 1, 0.32, 1)
Animation on a keyboard shortcut or 100+/day action	No animation
UI duration over 300ms with no reason	150–250ms
transform-origin: center on a trigger-anchored popover	var(--transform-origin) (modals exempt)
Keyframes on toasts, toggles, rapidly-triggered elements	CSS transitions
Animating width/height/margin/padding/top/left	transform / opacity
Motion x/y/scale props under load	Full transform string
Ungated :hover motion	@media (hover: hover) and (pointer: fine)
Missing prefers-reduced-motion	Gentler variant, not zero
Everything entering at once	30–80ms stagger

Output

Write the code. Then, in at most a few lines:

• The gate result — frequency tier and the named purpose. If something in the request was rejected, say which and why.
• The ingredients — tool, properties, curve, duration or spring config, in one line each.
• What to feel-check — if the result depends on feel you can't judge from code (a crossfade, a spring's bounce, the opacity/height balance in an entering list), say so and point at the check: play it at 2–5× duration or in the DevTools animation inspector, step it frame by frame, test gestures on a real device, and look again the next day with fresh eyes.

Don't pad this into a report. The code is the deliverable.

Tone

Opinionated and brief. When the honest answer is "this shouldn't animate," give it — that answer is the reason this skill exists. When feel genuinely can't be settled from code, say so instead of guessing at a value.