Transition	When to use	Reference
Card resize	Tween a container's width or height when its layout state changes.	01-card-resize.md
Number pop-in	Re-enter each digit with a blurred slide when a number updates.	02-number-pop-in.md
Notification badge	Slide a small badge onto a trigger and pop the dot.	03-notification-badge.md
Text states swap	Swap text in place with a blurred up-and-down transition.	04-text-states-swap.md
Menu dropdown	Open an origin-aware dropdown that grows from its trigger.	05-menu-dropdown.md
Modal open / close	Scale-up modal dialog with a softer scale-down on close.	06-modal.md
Panel reveal	Slide a panel into a region with a cross-blur.	07-panel-reveal.md
Page side-by-side	Slide between two side-by-side pages (list ↔ detail, step 1 ↔ step 2).	08-page-side-by-side.md
Icon swap	Cross-fade two icons in the same slot with blur and scale.	09-icon-swap.md
Success check	Compose fade + rotate + Y-bob + path stroke-draw to celebrate a completed action.	10-success-check.md
Avatar group hover	Distance-falloff lift on a row of items with a bouncy spring on return.	11-avatar-group-hover.md
Error state shake	Per-segment cubic-bezier shake with auto-reverting border + message.	12-error-state-shake.md

Decision rules

When the user asks for a transition, match against the visible UI element first, then the verb:

Trigger + small dot floating on top → notification badge.
Trigger + surface that grows from it → dropdown (anchored, origin-aware) or modal (centered, no anchor).
Surface that slides into a region of the page → panel reveal.
Two screens, list ↔ detail or step 1 ↔ step 2 → page side-by-side.
Element changes width or height → card resize.
Element's text content changes in place → text states swap.
Two icons in the same slot → icon swap.
A number updates → number pop-in.
Confirmation / success / "done" moment (checkmark, payment processed, file uploaded) → success check.
Hovering an item in a horizontal stack (avatars, chips, segmented buttons, tag pills) → avatar group hover.
Form validation error / "this is wrong" feedback (invalid field, wrong PIN, duplicate name) → error state shake.

If two transitions could fit, prefer the lower-overhead one (card resize over panel reveal, dropdown over modal, success check over a full modal celebration) unless the design clearly calls for the heavier surface. The success check is animation-only — if you also need to swap from a spinner to the check, pair it with icon swap.

Universal install

Drop this :root block into your project once. Every transition snippet reads from these semantic names — there are no per-component values to chase down later.

/* transitions-dev — copy this :root block into your project once.
   Every transition snippet reads from these semantic names. */
:root {
  /* Card resize */
  --resize-dur: 300ms;
  --resize-ease: cubic-bezier(0.22, 1, 0.36, 1);
  /* Number pop-in */
  --digit-dur: 500ms;
  --digit-distance: 8px;
  --digit-stagger: 70ms;
  --digit-blur: 2px;
  --digit-ease: cubic-bezier(0.34, 1.45, 0.64, 1);
  --digit-dir-x: 0;
  --digit-dir-y: 1;
  /* Notification badge */
  --badge-slide-dur: 260ms;
  --badge-pop-dur: 500ms;
  --badge-pop-close-dur: 180ms;
  --badge-fade-dur: 400ms;
  --badge-fade-close-dur: 180ms;
  --badge-blur: 2px;
  --badge-offset-x: -8.2px;
  --badge-offset-y: 12.4px;
  --badge-slide-ease: cubic-bezier(0.22, 1, 0.36, 1);
  --badge-pop-ease: cubic-bezier(0.34, 1.36, 0.64, 1);
  --badge-close-ease: cubic-bezier(0.4, 0, 0.2, 1);
  /* Text states swap */
  --text-swap-dur: 200ms;
  --text-swap-translate-y: 8px;
  --text-swap-blur: 2px;
  --text-swap-ease: ease-out;
  /* Menu dropdown */
  --dropdown-open-dur: 250ms;
  --dropdown-close-dur: 150ms;
  --dropdown-pre-scale: 0.97;
  --dropdown-closing-scale: 0.99;
  --dropdown-ease: cubic-bezier(0.22, 1, 0.36, 1);
  /* Modal open / close */
  --modal-open-dur: 250ms;
  --modal-close-dur: 150ms;
  --modal-scale: 0.96;
  --modal-scale-close: 0.96;
  --modal-ease: cubic-bezier(0.22, 1, 0.36, 1);
  /* Panel reveal */
  --panel-open-dur: 400ms;
  --panel-close-dur: 350ms;
  --panel-translate-y: 100px;
  --panel-blur: 2px;
  --panel-ease: cubic-bezier(0.22, 1, 0.36, 1);
  /* Page side-by-side */
  --page-slide-dur: 200ms;
  --page-fade-dur: 200ms;
  --page-slide-distance: 8px;
  --page-blur: 3px;
  --page-stagger: 0ms;
  --page-exit-enabled: 1;
  --page-slide-ease: cubic-bezier(0.22, 1, 0.36, 1);
  --page-fade-ease: cubic-bezier(0.22, 1, 0.36, 1);
  /* Icon swap */
  --icon-swap-dur: 200ms;
  --icon-swap-blur: 2px;
  --icon-swap-start-scale: 0.25;
  --icon-swap-ease: ease-in-out;
  /* Success check */
  --check-opacity-dur: 550ms;
  --check-rotate-dur: 550ms;
  --check-rotate-from: 80deg;
  --check-bob-dur: 450ms;
  --check-y-amount: 40px;
  --check-blur-dur: 500ms;
  --check-blur-from: 10px;
  --check-path-dur: 550ms;
  --check-path-delay: 80ms;
  --check-ease-out: cubic-bezier(0.22, 1, 0.36, 1);
  --check-ease-opacity: cubic-bezier(0.22, 1, 0.36, 1);
  --check-ease-rotate: cubic-bezier(0.22, 1, 0.36, 1);
  --check-ease-bob: cubic-bezier(0.34, 1.35, 0.64, 1);
  --check-ease-path: cubic-bezier(0.22, 1, 0.36, 1);
  /* Avatar group hover */
  --avatar-lift: -4px;
  --avatar-dur: 320ms;
  --avatar-scale: 1.05;
  --avatar-falloff: 0.45;
  --avatar-ease-in: cubic-bezier(0.22, 1, 0.36, 1);
  --avatar-ease-out: cubic-bezier(0.34, 3.85, 0.64, 1);
  /* Error state shake */
  --shake-distance: 6px;
  --shake-overshoot: 4px;
  --shake-dur-a: 80ms;
  --shake-dur-b: 60ms;
  --shake-ease: cubic-bezier(0.22, 1, 0.36, 1);
  --revert-hold: 3000ms;
  --revert-dur: 280ms;
}

The --pX-* source tokens used by the live demo at transitions.dev are intentionally not exported here. Tunable values are renamed to semantic names (--badge-*, --dropdown-*, --modal-*, …) so the user owns the design vocabulary.

Output format

When inserting a transition into the user's project:

Add the :root block above to the user's global stylesheet, but only if it isn't already there. If the user already imported the universal install block once, do not duplicate it.
Paste the chosen transition's CSS verbatim from the relevant reference file. Do not rewrite selectors, do not collapse the transition into shorthand, do not strip will-change. The snippets are tuned and tested.
Wire the documented HTML hooks — class names (.t-dropdown, .t-modal, .t-success-check, .t-avatar, .t-input, …) and state attributes (data-open, data-state, data-page, .is-open, .is-closing, .is-exit, .is-enter-start, .is-animating, .is-error, .is-shaking).
Preserve the @media (prefers-reduced-motion: reduce) block. Every snippet ships one. Removing it makes the component fail accessibility audits.
For transitions that need JS (dropdown, modal, text swap, number pop-in, page slide, success check, avatar group hover, error state shake), copy the small orchestration snippet from the reference file and adapt the selectors to the user's DOM. Keep the timing reads (getComputedStyle(...)getPropertyValue("--…")) so durations stay in sync with the :root values.

Keep the diff small: only edit the files needed to introduce the transition. Don't rename the user's existing variables, don't reformat unrelated CSS, don't pull in a motion library.

Common mistakes to avoid

Stripping the close-state class cleanup on dropdown/modal — without the setTimeout that removes .is-closing, the next open jumps from the closing scale instead of the resting pre-open scale.
Forgetting the reflow in the text swap, number pop-in, success check replay, and error state shake — void el.offsetWidth (or offsetHeight) between class/attribute removal and re-addition is what guarantees the animation replays.
Animating a single container instead of the inner pieces — for the badge, animate the dot, not the trigger; for page slide, animate the page sections, not the container.
Replacing transition: … with transition: all — every snippet enumerates exact properties on purpose so unrelated style changes don't ride in for free.
Hardcoding the success check's stroke-dasharray — the snippet ships 20 as a placeholder. Replace it with path.getTotalLength() rounded up by 1 for your path, otherwise the stroke pre-reveals or over-draws.
Setting transition-timing-function in CSS for the avatar group hover — it has to be set inline in JS before the --shift / --scale-active writes so the bouncy ease-out only applies on mouseleave.
Mixing .is-error and .is-shaking into one class for the error state shake — keeping them orthogonal is what allows the shake to replay (remove → reflow → re-add) without flickering the whole error treatment.

Reference files

01-card-resize.md — Card resize
02-number-pop-in.md — Number pop-in
03-notification-badge.md — Notification badge
04-text-states-swap.md — Text states swap
05-menu-dropdown.md — Menu dropdown
06-modal.md — Modal open / close
07-panel-reveal.md — Panel reveal
08-page-side-by-side.md — Page side-by-side
09-icon-swap.md — Icon swap
10-success-check.md — Success check
11-avatar-group-hover.md — Avatar group hover
12-error-state-shake.md — Error state shake
_root.css — the universal install block on its own, ready to import directly.

transitions-dev

Transitions.dev

Quick reference