Performant Web Animation

Eliminate the #1 cause of janky web animation: animating properties that force the browser to recalculate layout (reflow) or repaint on every frame. The browser renders in stages — layout → paint → composite. Animating width, height, top, left, margin, padding, box-shadow, or filter re-runs layout and/or paint each frame, blocking the main thread. Animating transform and opacity runs entirely on the compositor (often the GPU), skipping layout and paint, which is what makes animation smooth at 60/120fps.

When to use

Use when an animation stutters or drops frames, when a hover/scroll effect feels heavy, when animating size/position/shadow, when a layout change needs to animate smoothly (cards reordering, an element moving between containers), when animating height: auto, or when reviewing animation code for performance.

Core rule: animate only transform and opacity

Map every "expensive" animation to a cheap equivalent.

Animating (expensive)	Triggers	Replace with (cheap)
width / height	Layout + Paint	transform: scaleX()/scaleY() (+ FLIP for true size)
top / left / margin	Layout + Paint	transform: translate()
box-shadow	Paint	Animate opacity of a pseudo-element holding the shadow
filter: blur()	Paint (heavy)	Cross-fade two layers via opacity, or accept sparingly
background-position	Paint	transform: translate() on a child layer
color / background	Paint	Often acceptable; or cross-fade layers

Position and size via transform

/* BAD: animates layout every frame */
.box { transition: left 300ms, width 300ms; left: 0; width: 100px; }
.box:hover { left: 200px; width: 200px; }

/* GOOD: compositor-only */
.box {
  transition: transform 300ms ease;
  transform: translateX(0) scaleX(1);
  transform-origin: left center;
}
.box:hover { transform: translateX(200px) scaleX(2); }

scaleX distorts inner content (text stretches). For true resizing without distortion, use FLIP.

Cheap box-shadow via pseudo-element opacity

Animating box-shadow repaints a large blurred region every frame. Instead paint the shadow once on a ::after, then animate only its opacity.

.card { position: relative; }
.card::after {
  content: "";
  position: absolute;
  inset: 0;
  border-radius: inherit;
  box-shadow: 0 12px 28px rgba(0,0,0,0.35);
  opacity: 0;
  transition: opacity 300ms ease;
  pointer-events: none;
}
.card:hover::after { opacity: 1; }

The blurred shadow is rasterized once; hovering only changes a compositor opacity — smooth at any frame rate.

FLIP: animate layout changes cheaply

FLIP (First, Last, Invert, Play) animates a layout change (reorder, resize, move between containers) using only transform. Measure where the element was (First) and will be (Last), apply an inverting transform so it visually appears unmoved, then animate the transform back to identity (Play). The DOM ends in its real final layout; the motion is pure compositor work.

function flip(el, mutate) {
  const first = el.getBoundingClientRect();   // First
  mutate();                                    // change the DOM/layout
  const last = el.getBoundingClientRect();     // Last

  const dx = first.left - last.left;
  const dy = first.top - last.top;
  const sx = first.width  / last.width;
  const sy = first.height / last.height;

  el.animate(
    [
      { transformOrigin: 'top left',
        transform: `translate(${dx}px, ${dy}px) scale(${sx}, ${sy})` }, // Invert
      { transformOrigin: 'top left', transform: 'none' },               // Play
    ],
    { duration: 300, easing: 'cubic-bezier(0.2, 0, 0, 1)' }
  );
}

// usage: animate an element moving to a new grid position
flip(card, () => targetContainer.appendChild(card));

The Web Animations API animate() runs the transform on the compositor. For many elements, batch all getBoundingClientRect() reads before any mutation (see layout thrashing below).

Animating height: auto

height: auto is not interpolatable historically. Modern and fallback approaches:

/* Modern (Chrome 129+/supporting browsers): opt size keywords into animation */
.panel {
  interpolate-size: allow-keywords;   /* enables animating to/from auto */
  height: 0;
  overflow: clip;
  transition: height 300ms ease;
}
.panel.open { height: auto; }
/* calc-size() also works: height: calc-size(auto, size); */

/* Robust fallback today: CSS grid 1fr -> 0fr */
.wrapper {
  display: grid;
  grid-template-rows: 0fr;            /* collapsed */
  transition: grid-template-rows 300ms ease;
}
.wrapper.open { grid-template-rows: 1fr; }
.wrapper > .content { overflow: hidden; min-height: 0; }

The grid trick animates the track size (compositor-friendly enough and broadly supported) and needs no JS height measurement. Use interpolate-size: allow-keywords / calc-size(auto, size) where the target browsers support it; keep the grid technique as the cross-browser default.

Avoid layout thrashing (batch reads, then writes)

Reading a layout property (offsetWidth, getBoundingClientRect, scrollTop, getComputedStyle) after a write forces a synchronous reflow. Interleaving reads and writes in a loop ("layout thrashing") can run dozens of forced reflows per frame.

// BAD: read, write, read, write... forces reflow each iteration
items.forEach((el) => {
  const w = el.offsetWidth;          // read (forces layout)
  el.style.width = w * 1.5 + 'px';   // write (invalidates layout)
});

// GOOD: batch all reads, then all writes
const widths = items.map((el) => el.offsetWidth);  // all reads
items.forEach((el, i) => {                          // all writes
  el.style.width = widths[i] * 1.5 + 'px';
});

For frame-synced work, read in a requestAnimationFrame callback and apply writes; libraries like fastdom formalize this read/write scheduling.

will-change: use sparingly

will-change: transform promotes an element to its own compositor layer ahead of time, avoiding a hitch at animation start. But every promoted layer costs GPU memory, and over-use degrades performance.

.menu { will-change: transform; }   /* only on elements about to animate */

Rules: apply just before the animation (e.g. on hover/parent state), remove it after (will-change: auto) when idle, never blanket-apply to many elements, and never leave it permanently on large/numerous nodes. A single transform: translateZ(0) hack does the same promotion but is harder to undo — prefer will-change.

Deliver & verify (standalone HTML)

Packaged helper (scripts/): scripts/seek-shot.sh anim.html 0 1.5 3 freezes the ?t=N harness and screenshots each moment; scripts/contact-sheet.sh sheet.png frame-*.png tiles them for one-glance review. See scripts/README.md.

The deliverable is one self-contained .html that opens directly in a browser — the markup, the CSS/JS animation, and a freeze harness in one file. For this skill, verification is two-pronged: the frame must look right and be cheap to produce (compositor-only).

Output contract:

• One .html, deps via CDN if any, animation driven by CSS transitions/@keyframes or the Web Animations API.
• A freeze mechanism so a screenshot lands on a deterministic frame:
  • CSS @keyframes → ?t=N sets el.style.animationDelay = (-N)+'s'; el.style.animationPlayState = 'paused'.
  • WAAPI / JS → keep the animation object and anim.pause(); anim.currentTime = N*1000.

Verify loop — freeze → screenshot, then profile for jank:

1. Open headless at start / mid / end (?t=0, mid, end), screenshot each; confirm the motion is visually correct (no clipped/stretched text from scaleX, FLIP lands on the real layout, shadow/height transitions look right).
2. Confirm it's compositor-only — the whole point of this skill. Either:
   • DevTools → Performance: record the animation, check there is no purple "Layout" or green "Paint" band per frame (only "Composite Layers").
   • Or headless trace: npx playwright screenshot for the visual, plus a CDP/tracing capture, and assert no Layout/Paint events fire during the animation window.
3. Watch the FPS/“Frame Rendering Stats” overlay stays at 60 — dropped frames mean an expensive property slipped back in.

npx playwright screenshot --wait-for-timeout=500 "file://$PWD/anim.html?t=0.3" frame.png

Before you finish:

1. Opens standalone — no console errors.
2. Only transform / opacity animate per frame; DevTools shows no per-frame Layout/Paint.
3. Screenshotted at start / mid / end — correct, no scaleX text distortion, FLIP lands true.
4. Holds 60fps; will-change applied just-in-time and removed when idle (no leftover layers).
5. prefers-reduced-motion honored.

Quick reference

Goal	Do this
Move element	transform: translate()
Resize without distortion	FLIP with transform
Shadow on hover	Animate opacity of shadow pseudo-element
Expand to content height	grid 0fr→1fr, or interpolate-size: allow-keywords
Many elements moving	Batch getBoundingClientRect reads, then writes
Smooth animation start	will-change just-in-time, remove when idle
Verify it's compositor-only	DevTools Performance: no purple "Layout"/green "Paint" per frame

Gotchas

• scaleX/scaleY stretches text and children; use FLIP when content must stay crisp.
• transform percentages are relative to the element's own box, not the parent — different from left: %.
• Overusing will-change or translateZ(0) creates too many layers and hurts performance; promote only what animates.
• filter and backdrop-filter are compositor-related but still expensive; animate their presence via opacity cross-fades rather than animating the blur radius.
• The grid 1fr→0fr content must have overflow: hidden and min-height: 0 or it won't collapse.
• Always gate non-essential motion behind @media (prefers-reduced-motion: reduce).

Reference files

• references/patterns-and-profiling.md — full runnable examples (accordion, reorder list, parallax), DevTools profiling walkthrough to confirm compositor-only frames, reduced-motion patterns, and a property-cost cheat sheet.