Transitions.dev

Nine portable CSS transitions, each namespaced under t-* selectors with semantic CSS custom properties. Drop-in: paste the snippet, wire the documented HTML hooks, done. No framework dependencies, no demo-specific markup, and every snippet ships a prefers-reduced-motion guard.

Quick reference

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

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.

If two transitions could fit, prefer the lower-overhead one (card resize over panel reveal, dropdown over modal) unless the design clearly calls for the heavier surface.

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;
}

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, …) and state attributes (data-open, data-state, data-page, .is-open, .is-closing, .is-exit, .is-enter-start, .is-animating).
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), 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 and number pop-in — void el.offsetHeight between class 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.

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
_root.css — the universal install block on its own, ready to import directly.