CSS Scroll-driven Animationsの使い方【scroll・view・読了バー】

<div class="reading-progress" aria-hidden="true"></div>

<main class="article-shell">
  <img class="nines-logo" src="https://kjnine.com/wp-content/themes/kjnines_theme/images/svg2/nines_logo.svg" alt="NINES">
  <header class="hero">
    <p class="eyebrow">SCROLL-DRIVEN ANIMATIONS</p>
    <h1>スクロール量に連動する記事ページ</h1>
    <p>上部の進捗バーと、画面に入るカードの動きをCSSだけで作ります。</p>
  </header>

  <section class="feature-list">
    <article class="feature-card"><span>01</span><h2>scroll()</h2><p>スクロールコンテナの進行量を使います。</p></article>
    <article class="feature-card"><span>02</span><h2>view()</h2><p>要素が表示領域へ入る進行量を使います。</p></article>
    <article class="feature-card"><span>03</span><h2>animation-range</h2><p>動き始めと終わりの範囲を調整します。</p></article>
    <article class="feature-card"><span>04</span><h2>Fallback</h2><p>未対応でも内容は常に読める状態にします。</p></article>
  </section>
</main>

* {
  box-sizing: border-box;
}

html {
  color: #111;
  background: #f2f5f9;
  font-family: system-ui, sans-serif;
}

body {
  min-height: 220vh;
  margin: 0;
}

.reading-progress {
  position: fixed;
  z-index: 10;
  inset: 0 0 auto;
  height: 5px;
  background: #0057ff;
  transform: scaleX(0);
  transform-origin: left;
  animation: reading-progress linear both;
  animation-timeline: scroll(root block);
  animation-range: 0% 100%;
}

@keyframes reading-progress {
  to {
    transform: scaleX(1);
  }
}

.article-shell {
  position: relative;
  width: min(100% - 32px, 920px);
  margin: 0 auto;
  padding: 80px 0 160px;
}

.nines-logo {
  position: absolute;
  top: 32px;
  right: 0;
  width: 68px;
  opacity: 0.14;
}

.hero {
  min-height: 65vh;
  display: grid;
  align-content: center;
}

.eyebrow,
.feature-card span {
  color: #0057ff;
  font-size: 0.78rem;
  font-weight: 800;
  letter-spacing: 0.14em;
}

.hero h1 {
  max-width: 760px;
  font-size: clamp(2.4rem, 8vw, 5.5rem);
  line-height: 1.05;
}

.feature-list {
  display: grid;
  gap: 28px;
}

.feature-card {
  min-height: 300px;
  display: grid;
  align-content: center;
  padding: clamp(28px, 6vw, 64px);
  border-left: 5px solid #0057ff;
  background: #fff;
  box-shadow: 0 18px 50px rgb(20 35 65 / 8%);
}

@supports (animation-timeline: view()) {
  .feature-card {
    animation: card-in linear both;
    animation-timeline: view();
    animation-range: entry 15% cover 35%;
  }

  @keyframes card-in {
    from {
      opacity: 0;
      transform: translateY(48px);
    }
    to {
      opacity: 1;
      transform: translateY(0);
    }
  }
}

@media (prefers-reduced-motion: reduce) {
  .reading-progress,
  .feature-card {
    animation: none;
    transform: none;
    opacity: 1;
  }
}

// このデモはCSSだけで動きます。
// 対応確認が必要な場合はCSS.supports()を利用できます。

const supported = CSS.supports('animation-timeline: scroll()');
console.log({ scrollDrivenAnimations: supported });