一貫したクロスドキュメント遷移

/*
  MANDATORY: Include this rule in every page that participates
  in cross-document view transitions.
  `navigation: auto` enables transitions for standard navigations
  (link clicks, form submissions, back/forward).
*/
@view-transition {
  navigation: auto;
}

/* MANDATORY Copy-Paste Safety: Disable cross-document view transitions for users requesting reduced motion */
@media (prefers-reduced-motion: reduce) {
  @view-transition {
    navigation: none;
  }
}

<head>
  <!--
    DO: Mark layout-critical scripts with blocking="render".
  -->
  <script type=module blocking="render">
    // Example: apply a stored theme before the page renders,
    // so the transition snapshot reflects the correct theme.
    document.documentElement.dataset.theme =
      localStorage.getItem('theme') || 'light';
  </script>
</head>

<head>
  <link rel="stylesheet" href="/css/styles.css">

  <!--
    DO: Block rendering until the main content area is parsed,
    even for a simple cross-fade. Without this, the browser may
    snapshot the page before visible content exists in the DOM,
    causing the cross-fade to reveal a blank or partial page.
  -->
  <link rel="expect" href="#main-content" blocking="render">
</head>
<body>
  <header>...</header>
  <main id="main-content">
    <h1>Page Title</h1>
    <p>Above-the-fold content the user should see immediately.</p>
  </main>
  <!-- Content below the fold does NOT need to be blocked on -->
  <section>...</section>
</body>

<head>
  <link rel="stylesheet" href="/css/styles.css">

  <!--
    DO: Preload critical transition images so their pixels are
    available to decode before the new-page snapshot is captured.
    <link rel="expect"> only waits for the DOM element to be parsed,
    NOT for image pixels to download or decode.
  -->
  <link rel="preload" as="image" href="/img/product.webp" fetchpriority="high">

  <!--
    DO: Block rendering until the element participating in the
    morph animation has been parsed. Without this, the browser
    may start the transition before #hero exists, causing the
    morph to degrade to a fade-out/fade-in.
  -->
  <link rel="expect" href="#hero" blocking="render">

  <!--
    When multiple blocking="render" resources are present,
    rendering is blocked until ALL of them are satisfied.
    Here, the browser waits for both the script to execute
    AND the #hero element to be parsed — whichever comes last.
  -->
  <script async blocking="render" src="/js/transition-setup.js"></script>
</head>
<body>
  <header>...</header>
  <section id="hero">
    <h1 style="view-transition-name: page-title">Product Name</h1>
    <!--
      DO: Provide explicit dimensions (width/height or aspect-ratio)
      as a sizing fallback so the snapshot captures the correct layout
      box even if image decoding is still in progress.
    -->
    <img
      style="view-transition-name: hero-image"
      src="/img/product.webp"
      width="800"
      height="600"
      alt="Product"
    >
  </section>
</body>

<head>
  <!--
    DO: Use media queries to conditionally block rendering.
    On wide screens, both the hero and the sidebar are visible,
    so block until both are parsed. On narrow screens, only the
    hero is visible initially.
  -->
  <link
    rel="expect"
    href="#hero"
    blocking="render"
    media="screen and (width <= 768px)"
  >
  <link
    rel="expect"
    href="#sidebar"
    blocking="render"
    media="screen and (width > 768px)"
  >
</head>

<head>
  <!--
    MANDATORY: The pagereveal listener must be registered before
    the page renders. Inline small setup scripts with blocking="render"
    to avoid an extra network round-trip on cold loads, or use an
    external async script with blocking="render" so the listener is
    registered early without blocking HTML parsing.
    If the listener is registered too late (e.g., in a deferred
    script), the event may have already fired.
  -->
  <script async blocking="render" src="/js/transition-setup.js"></script>
</head>

// transition-setup.js
window.addEventListener('pagereveal', async (event) => {
  if (!event.viewTransition) return;

  const from = navigation.activation?.from;
  if (!from) return;

  const fromUrl = new URL(from.url);

  // DO: Assign view-transition-name based on navigation context.
  // This enables a morph animation from the product card on the
  // list page to the heading on the detail page.
  if (fromUrl.pathname === '/products/') {
    const heading = document.querySelector('main h1');
    if (!heading) return;

    heading.style.viewTransitionName = 'product-title';

    try {
      await event.viewTransition.ready;
    } catch (error) {
      // Skipped view transitions reject `ready` with InvalidStateError.
      if (error.name !== 'InvalidStateError') {
        throw error;
      }
    } finally {
      // MANDATORY: Remove the temporary name after the transition
      // finishes. Stale names interfere with subsequent navigations
      // and prevent the page from entering the bfcache.
      await event.viewTransition.finished;
      heading.style.viewTransitionName = '';
    }
  }
});