Transitions Dev

Transitions.dev

Twenty-one 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

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.
• Clearing a text field (search box × button, filter reset) → input clear with dissolve.
• Placeholder that loads then swaps to real content (list row, card, profile header) → skeleton loader and reveal.
• In-progress / "thinking" text that should feel alive (loading label, streaming status) → shimmer text.
• Small horizontal set of mutually-exclusive options with a moving highlight (view switcher, segmented control, filter tabs) → tabs sliding.
• Hover/focus hint that appears over a trigger (icon tooltip, info bubble) → tooltip open / close.
• Stacked headline + supporting line entering with rhythm (hero copy, empty state, onboarding step) → texts reveal.
• Card / tile that should react in 3D to the pointer on hover (product card, cover art, membership card, with or without a light glare) → card hover tilt.
• Circular trigger that becomes the surface it opens (+ FAB grows into a menu / panel, compose button expands) → plus → menu morph. If the surface is a separate popover that merely grows from the trigger, use menu dropdown instead.
• Header with a collapsible body that grows / shrinks in height (settings group, FAQ, filter section, "show more", disclosure) → accordion expand.
• No clear match → fall back to transitions reveal and let the user pick. Don't guess.

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.

Commands

The skill exposes two namespaced verbs the agent should recognise in addition to direct transition requests. Every command starts with transitions so the invocation never collides with verbs from other skills installed in the same project. (For auditing or applying motion-token polish on transitions that already exist, the companion transitions-polish skill provides transitions review and transitions polish.)

transitions reveal — list every transition

Trigger phrases: transitions reveal, "reveal the transitions", "list all transitions", "what transitions are in this skill", "show the transitions catalog".

Behaviour: print the twenty-one transitions as a numbered plain-text list — name, one-line summary, and the matching reference filename. Reuse the rows in ## Quick reference above; do not invent new copy. No project access.

transitions apply — install the best-fit transition

Trigger phrases: transitions apply, "apply a transition here", "add the right transition", "install transitions-dev here", "fix the animation on this element".

Behaviour:

1. Read context: the currently-open file, the element nearest the cursor, surrounding CSS / JSX. If the user named a transition explicitly (e.g. transitions apply menu-dropdown), use it.
2. Run the decision rules from ## Decision rules on that context and pick one transition. If two could fit, prefer the lower-overhead one (same tie-breaker the existing rules use).
3. Surface a one-line proposal: "I'd apply menu-dropdown here because the element opens from a trigger and is anchored. Confirm to install?".
4. On confirmation, follow the existing five-step procedure in ## Output format verbatim (root block, snippet, hooks, reduced-motion guard, JS orchestration if needed).
5. If the agent can't pick a single transition with confidence, fall back to transitions reveal and ask the user to choose.

Universal install

Copy _root.css into your project once and import it (or paste its :root block into your global stylesheet). It holds the semantic tunable variables for all twenty-one transitions. Every snippet reads from these names — --resize-*, --badge-*, --dropdown-*, --clear-*, --shimmer-*, --tabs-*, --tt-*, --stagger-*, --tilt-*, --morph-*, --acc-*, and the rest. The snippets ship literal values, so they work on their own; to tune them against a shared motion-token scale, install the companion transitions-polish skill.

Each reference file also restates just the variables that snippet needs, so you can install a single transition without pulling the whole block. Don't duplicate the block — if _root.css is already imported, skip re-pasting any per-snippet :root.

The --pX-* source tokens used by the live demo at transitions.dev are intentionally not exported. Tunable values are renamed to semantic names so the user owns the design vocabulary. A few transitions (input clear, shimmer text, tabs, tooltip) carry color tokens that differ by theme — each reference file documents the html[data-theme="dark"] overrides.

Output format

When inserting a transition into the user's project:

1. Install the variables from _root.css into the user's global stylesheet, but only if they aren't already there — or just the per-snippet :root block from the reference file if installing a single transition. If the universal block is already imported, do not duplicate it.
2. 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.
3. Wire the documented HTML hooks — class names (.t-dropdown, .t-modal, .t-success-check, .t-avatar, .t-clear, .t-skel, .t-shimmer, .t-tabs, .t-tt, .t-stagger, .t-tilt, .t-morph, .t-acc, …) and state attributes (data-open, data-state, data-page, data-origin, aria-selected, aria-expanded, .is-open, .is-closing, .is-error, .is-shaking, .has-value, .is-clearing, .is-pulsing, .is-revealed, .is-shown, .is-hiding, .is-hover, .is-tilting).
4. Preserve the @media (prefers-reduced-motion: reduce) block. Every snippet ships one. Removing it makes the component fail accessibility audits.
5. For transitions that need JS (dropdown, modal, text swap, number pop-in, page slide, success check, avatar group hover, error state shake, input clear, skeleton reveal, tabs sliding, texts reveal, card hover tilt, plus → menu morph, accordion expand), 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. Shimmer text and tooltip are pure CSS — no JS needed.

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.
• Leaving the input clear glow on mix-blend-mode: multiply in dark mode — flip to screen, bump --glow-opacity to ~0.85, and paint white gradients in JS.
• Forgetting to write the tabs pill's first position without a transition — on first paint and resize, set transform + width with transition: none (then reflow + restore) or the pill animates in from translateX(0) / width: 0.
• Tracking the pointer on the tilting element itself for card hover tilt — bind pointermove to the flat outer .t-tilt wrapper, not .t-tilt-card, or the rotating edges slip under the cursor and the hover flickers.
• Padding on the accordion grid track — put padding on .t-acc-panel-inner, never on .t-acc-panel; padding on the 0fr track leaves a residual height strip so the panel never fully closes.
• Morphing the accordion chevron's d path — CSS d: path interpolation is Chromium-only, so it never animates on mobile Safari / Firefox. Flip the chevron vertically (transform: scaleY(-1)) instead — it passes through a flat line at the midpoint just like the path morph and works everywhere. Keep the path symmetric about its viewBox centre and add vector-effect: non-scaling-stroke so the stroke stays constant through the flip. This is what the snippet ships.

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
• 13-input-clear-dissolve.md — Input clear with dissolve
• 14-skeleton-reveal.md — Skeleton loader and reveal
• 15-shimmer-text.md — Shimmer text
• 16-tabs-sliding.md — Tabs sliding
• 17-tooltip.md — Tooltip open/close
• 18-texts-reveal.md — Texts reveal
• 19-card-tilt.md — Card hover tilt
• 20-plus-menu-morph.md — Plus to menu morph
• 21-accordion.md — Accordion expand
• _root.css — the universal install block on its own, ready to import directly.