agents-inc/web-animation-view-transitions

View Transitions API Patterns

Quick Guide: Use the View Transitions API for native page/state transitions. document.startViewTransition() for same-document, @view-transition { navigation: auto } for cross-document MPA. Always feature-detect before use and respect prefers-reduced-motion. Use the options form startViewTransition({ update, types }) when you need typed transitions.

---

<critical_requirements>

CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST feature-detect before using startViewTransition - it is NOT available in all browsers)

(You MUST respect prefers-reduced-motion by providing reduced or disabled animations)

(You MUST ensure view-transition-name values are unique - duplicate names break transitions)

(You MUST clean up dynamically assigned view-transition-name values after transitions complete)

(You MUST use named constants for all animation timing values - NO magic numbers)

</critical_requirements>

---

Auto-detection: View Transitions API, startViewTransition, view-transition-name, @view-transition, ::view-transition, pageswap, pagereveal, ViewTransition, view-transition-class, match-element, active-view-transition-type

When to use:

• Animating state changes in single-page applications
• Creating smooth page-to-page transitions in multi-page applications
• Implementing shared element (hero) animations between views
• Providing visual continuity during navigation
• Creating custom transition effects (slide, scale, circular reveal)

Key patterns covered:

• Same-document transitions with startViewTransition()
• Cross-document MPA transitions with @view-transition CSS
• view-transition-name for shared element animations
• Pseudo-element styling (::view-transition-old, ::view-transition-new)
• Direction-aware transitions with :active-view-transition-type()
• Feature detection and graceful fallbacks
• prefers-reduced-motion accessibility patterns

When NOT to use:

• Complex physics-based animations (use animation libraries)
• Animations requiring precise timeline control
• Simple hover/focus effects (use CSS transitions)

Detailed Resources:

• examples/core.md - Feature detection, state transitions, promise handling, CSS customization
• examples/spa.md - Theme switcher, form steps, tab panels, list reordering
• examples/shared-elements.md - Hero animations, multiple shared elements, MPA shared elements, modals
• reference.md - Decision frameworks, pseudo-element reference, browser support, anti-patterns

---

<philosophy>

Philosophy

The View Transitions API provides a native browser mechanism for creating animated transitions between DOM states or pages. It captures "before" and "after" snapshots, overlays them as pseudo-elements, and animates between them.

Core principles:

1. Native over library - Browser-native transitions are more performant and require less JavaScript
2. Progressive enhancement - Always feature-detect and provide functional fallback
3. Snapshot-based - Old state is captured as a screenshot, new state as a live representation
4. CSS-driven - Customize animations through pseudo-element CSS, not JavaScript
5. Accessibility-first - Always respect prefers-reduced-motion user preferences

</philosophy>

---

<patterns>

Core Patterns

Pattern 1: Feature Detection with Fallback

Always check for API support before using View Transitions. See examples/core.md Pattern 1 for full utility.

const SUPPORTS_VIEW_TRANSITIONS =
  typeof document !== "undefined" && "startViewTransition" in document;

function updateWithTransition(updateFn: () => void | Promise<void>): void {
  if (!SUPPORTS_VIEW_TRANSITIONS) {
    updateFn();
    return;
  }
  document.startViewTransition(() => updateFn());
}

Why good: Prevents runtime errors in unsupported browsers, provides seamless fallback

---

Pattern 2: Same-Document (SPA) Transitions

Animate DOM state changes within a single page. See examples/core.md Patterns 2-5 for state transitions, async loading, promise handling, and skip logic.

// startViewTransition accepts a callback or an options object
const transition = document.startViewTransition(async () => {
  await updateFn();
});

// Options form - set types for CSS targeting
const transition = document.startViewTransition({
  update: () => updateDOM(),
  types: ["slide-forward"],
});

await transition.finished;

ViewTransition object provides three promises:

| Promise | Resolves when | | ------------------------------- | ----------------------------- | | transition.ready | Pseudo-element tree created | | transition.updateCallbackDone | DOM update callback completed | | transition.finished | Animation complete |

---

Pattern 3: Cross-Document (MPA) Transitions

Enable transitions between separate pages without JavaScript. Both pages must opt in.

/* Include on BOTH source and destination pages */
@view-transition {
  navigation: auto;
}

Why good: No JavaScript required, works for traverse/push/replace navigations

Obsolete syntax: <meta name="view-transition" content="same-origin"> - use the CSS at-rule instead.

---

Pattern 4: Shared Element Transitions

Create hero animations by giving matching elements the same view-transition-name. See examples/shared-elements.md for full product list-to-detail, multi-element, and MPA examples.

:root {
  --hero-duration: 300ms;
  --hero-easing: cubic-bezier(0.4, 0, 0.2, 1);
}

/* Same name on both pages/states creates shared element animation */
.product-thumbnail {
  view-transition-name: product-hero;
}
.product-image {
  view-transition-name: product-hero;
}

::view-transition-group(product-hero) {
  animation-duration: var(--hero-duration);
  animation-timing-function: var(--hero-easing);
}

Key rules: Names must be unique across the document. Clean up dynamically assigned names after transition.finished.

---

Pattern 5: Custom CSS Animations

Override default cross-fade with custom animations via pseudo-elements. See examples/core.md Pattern 6 for full examples.

:root {
  --transition-duration: 300ms;
  --transition-easing: ease-in-out;
}

::view-transition-old(root) {
  animation: slide-out-left var(--transition-duration) var(--transition-easing);
}
::view-transition-new(root) {
  animation: slide-in-right var(--transition-duration) var(--transition-easing);
}

Why good: CSS custom properties for timing constants, GPU-accelerated transforms

---

Pattern 6: Direction-Aware Transitions

Use different animations for forward vs backward navigation. Use the types parameter or ViewTransition.types set.

html:active-view-transition-type(forwards) {
  &::view-transition-old(content) {
    animation-name: slide-out-left;
  }
  &::view-transition-new(content) {
    animation-name: slide-in-right;
  }
}

html:active-view-transition-type(backwards) {
  &::view-transition-old(content) {
    animation-name: slide-out-right;
  }
  &::view-transition-new(content) {
    animation-name: slide-in-left;
  }
}

// Preferred: set types via options parameter
document.startViewTransition({
  update: () => navigateForward(),
  types: ["forwards"],
});

// Alternative: mutate types set on existing transition
const transition = document.startViewTransition(updateFn);
transition.types.add("forwards");

See examples/spa.md for form step and tab panel examples.

---

Pattern 7: Accessibility - Reduced Motion

Always respect user preferences for reduced motion.

@media (prefers-reduced-motion: reduce) {
  ::view-transition-group(*),
  ::view-transition-old(*),
  ::view-transition-new(*) {
    animation-duration: 0.01ms !important;
  }
}

const REDUCED_MOTION_QUERY = "(prefers-reduced-motion: reduce)";

function shouldEnableTransitions(): boolean {
  if (window.matchMedia(REDUCED_MOTION_QUERY).matches) return false;
  return "startViewTransition" in document;
}

See examples/spa.md for a full accessible transition wrapper with preference change monitoring.

---

Pattern 8: Circular Reveal Effect

Advanced custom animation using Web Animations API. Must await transition.ready before animating pseudo-elements.

const REVEAL_DURATION_MS = 400;
const REVEAL_EASING = "ease-in-out";

const transition = document.startViewTransition(updateFn);
await transition.ready;

document.documentElement.animate(
  {
    clipPath: [`circle(0 at ${x}px ${y}px)`, `circle(${r}px at ${x}px ${y}px)`],
  },
  {
    duration: REVEAL_DURATION_MS,
    easing: REVEAL_EASING,
    pseudoElement: "::view-transition-new(root)",
  },
);

See examples/spa.md for a complete theme-switcher circular reveal implementation.

</patterns>

---

<red_flags>

RED FLAGS

High Priority Issues:

• Missing feature detection - Calling startViewTransition() without checking support crashes in older browsers
• Duplicate view-transition-name values - Two visible elements with the same name breaks the transition entirely
• Not cleaning up dynamic names - Leftover names cause conflicts in subsequent transitions
• Ignoring prefers-reduced-motion - Mandatory for accessibility; always provide reduced or no animation
• Magic numbers for timing - All duration/delay values must be named constants or CSS custom properties

Medium Priority Issues:

• Using obsolete meta tag syntax - <meta name="view-transition"> is deprecated; use @view-transition CSS
• Not awaiting transition.ready for custom animations - Web Animations API must wait for pseudo-elements to exist
• Missing @view-transition on both MPA pages - Cross-document transitions require opt-in on source AND destination
• Setting view-transition-name in CSS for dynamic lists - Causes name conflicts; use JavaScript assignment or match-element

Gotchas & Edge Cases:

• Old state is a screenshot - Videos, animations, GIFs freeze in the old snapshot
• New state is "live" - Interactive content continues playing in the new snapshot
• Transition names are global - Same name on different page sections will conflict
• Animations block interaction - User cannot interact until transition completes; keep animations under 300ms
• Cross-document needs same-origin - Different origins cannot share transitions
• match-element requires Chrome 137+/Safari 18.4+ - Not yet available in Firefox
• pagereveal must be registered early - Put handler in <head> or use blocking="render"
• Reserved names (auto, inherit, none, unset) are CSS keywords, not valid custom identifiers

</red_flags>

---

<critical_reminders>

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md

(You MUST feature-detect before using startViewTransition - it is NOT available in all browsers)

(You MUST respect prefers-reduced-motion by providing reduced or disabled animations)

(You MUST ensure view-transition-name values are unique - duplicate names break transitions)

(You MUST clean up dynamically assigned view-transition-name values after transitions complete)

(You MUST use named constants for all animation timing values - NO magic numbers)

Failure to follow these rules will break transitions in unsupported browsers and create inaccessible experiences.

</critical_reminders>