コンテナースクロール状態クエリーの使用

.container {
  container-type: scroll-state;
  container-name: my-container;
}

@container my-container scroll-state(scrollable: top) {
  /* CSS ルールをここに置く */
}

<a class="back-to-top" href="#" aria-label="Top of page">↑</a>
<article>
  <h1>コンテナークエリーで制御する「先頭へ戻る」リンクのついたリーダー</h1>
  <section>
    <header>
      <h2>この最初の節は興味深い</h2>

      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</p>
    </header>

    ...
  </section>

  ...
</article>

<a class="back-to-top" href="#" aria-label="Scroll back to top">↑</a>
<article>
  <h1>コンテナークエリーで制御する「先頭へ戻る」リンクのついたリーダー</h1>
  <section>
    <header>
      <h2>この最初の節は興味深い</h2>

      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</p>
    </header>

    <p>
      Mauris non malesuada est, sed vestibulum nibh. Duis vestibulum iaculis
      lectus, eu sagittis dolor dignissim iaculis. Nunc et orci sed sapien
      eleifend placerat. Curabitur dapibus risus eget odio sollicitudin, sit
      amet luctus justo pellentesque.
    </p>

    <p>
      <strong>Morbi non pharetra quam.</strong> Fusce vestibulum sem diam, ac
      consequat augue consectetur ut. Donec at augue viverra, tempus urna sit
      amet, porta augue.
      <em>Phasellus fringilla tincidunt sem ullamcorper varius.</em> Aenean
      gravida feugiat sem nec ultricies.
    </p>

    <img src="#" alt="Placeholder" />

    <p>
      Sed pellentesque placerat mi sed maximus. Sed vitae dui vitae mi pulvinar
      gravida sed et libero.
      <a href="#">Duis nec venenatis dolor, sed tristique felis.</a>
      Integer dapibus facilisis leo elementum vulputate. Curabitur a urna quis
      nulla vulputate tincidunt quis ac enim.
    </p>

    <p>
      Cras non elit vel leo dignissim convallis. Duis eros urna, varius sit amet
      lorem vel, feugiat euismod est.
      <strong>Aliquam ornare eu elit ut iaculis.</strong>
      Suspendisse vulputate tempor leo, non rhoncus risus aliquam vel.
    </p>
  </section>
  <section>
    <header>
      <h2>これはそれほどではありません</h2>

      <p>Suspendisse varius est ac turpis mollis cursus.</p>
    </header>

    <p>
      <strong
        >Curabitur faucibus condimentum eros, ut auctor felis lacinia
        sed.</strong
      >
      Praesent vitae scelerisque eros.
    </p>

    <p>
      <em>Ut vitae suscipit augue.</em> Cras et orci condimentum ante dignissim
      iaculis. Sed consectetur quis est sed dignissim. Nulla egestas orci erat,
      et commodo arcu feugiat ut.
    </p>

    <img src="#" alt="Placeholder" />

    <p>
      Sed non tempor massa, at accumsan ante. Pellentesque habitant morbi
      <a href="#">tristique senectus</a> et netus et malesuada fames ac turpis
      egestas.
    </p>

    <p>
      Pellentesque placerat luctus tempor. Nunc congue dapibus eros, at
      vulputate nulla. Sed rutrum eleifend magna vel porta. Integer cursus orci
      faucibus turpis scelerisque, nec pharetra arcu molestie.
    </p>
  </section>
  <section>
    <header>
      <h2>この節で、多少なりとも不明点が解消されたでしょうか？</h2>

      <p>Curabitur facilisis ornare lorem et eleifend.</p>
    </header>

    <p>
      <strong>Aenean mollis non neque sed finibus.</strong> Lorem ipsum dolor
      sit amet, consectetur adipiscing elit. Suspendisse sagittis viverra urna.
      In hac habitasse platea dictumst. Vestibulum neque orci, mollis sagittis
      augue et, pharetra vehicula diam.
    </p>

    <img src="#" alt="Placeholder" />

    <p>
      <a href="#">Pellentesque sollicitudin</a> nunc quis nisl condimentum, ac
      iaculis libero feugiat.
      <strong>Nullam ultrices purus a nulla dignissim hendrerit.</strong> In
      molestie consectetur est quis pulvinar.
    </p>

    <p>
      Vivamus ac erat eu est lobortis commodo. Orci varius natoque penatibus et
      magnis dis parturient montes, nascetur ridiculus mus. In nulla turpis,
      <strong>mollis et est tempor</strong>, dignissim aliquam metus. Proin eu
      arcu quis erat mollis pulvinar. Vivamus at facilisis neque.
    </p>

    <p>
      Integer bibendum laoreet erat, quis vulputate mauris bibendum nec. Class
      aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos
      himenaeos. Nam ut est in arcu interdum hendrerit.
    </p>
  </section>
  <section>
    <header>
      <h2>これらのまとめ</h2>

      <p>Nunc facilisis augue quis ex porta aliquam.</p>
    </header>

    <img src="#" alt="Placeholder" />

    <p>
      <strong
        >Fusce nisi enim, venenatis a est vel, varius placerat lacus.</strong
      >
      Nunc tempus rutrum nisl bibendum aliquet. Pellentesque vitae nunc sed nisl
      tincidunt elementum a sit amet nisi. Morbi pretium at dolor in pulvinar.
      Curabitur dapibus eleifend accumsan.
    </p>

    <p>
      Donec rhoncus, leo vitae mollis maximus, tellus lorem interdum arcu, eu
      <em>tempor lectus libero in risus</em>. Ut sit amet magna vitae mauris
      tempor bibendum. <a href="#">Integer id mauris ut ex mattis finibus.</a>
    </p>

    <p>
      Curabitur dui felis, elementum et tellus id, blandit facilisis lorem.
      Aliquam sed posuere ligula, at auctor ipsum. Morbi dignissim accumsan
      tellus pretium iaculis.
    </p>
  </section>
</article>

/* 全般的なスタイル設定 */

* {
  box-sizing: border-box;
}

html {
  font-family: Arial, Helvetica, sans-serif;
  height: 100%;
}

body {
  height: inherit;
  width: 100%;
  max-width: 540px;
  padding: 20px;
  margin: 0 auto;
}

p {
  line-height: 1.5;
}

img {
  display: block;
  width: 90%;
  margin: 30px auto;
  padding: 20px;
  border: 2px solid gray;
  aspect-ratio: 3/2;
}

.back-to-top {
  width: 64px;
  height: 64px;
  color: white;
  text-align: center;
  position: fixed;
  bottom: 10px;
  right: 10px;
  translate: 80px 0;
  transition:
    0.4s translate,
    0.2s background-color;
}

.back-to-top {
  text-decoration: none;
  border-radius: 50%;
  border: 1px solid #0007;
  background-color: #0007;
  color: white;
  font-size: 3rem;
  text-shadow: 0 0 2px black;
  padding-bottom: 10px;
}

.back-to-top:hover,
.back-to-top:focus {
  background: #0009;
}

html {
  container-type: scroll-state;
  container-name: scroller;
}

@container scroller scroll-state(scrollable: top) {
  .back-to-top {
    translate: 0 0;
  }
}

<main>
  <section>
    <div class="wrapper">
      <h2>Section 1</h2>
    </div>
  </section>

  ...
</main>

<main>
  <section>
    <div class="wrapper">
      <h2>Section 1</h2>
    </div>
  </section>
  <section>
    <div class="wrapper">
      <h2>Section 2</h2>
    </div>
  </section>
  <section>
    <div class="wrapper">
      <h2>Section 3</h2>
    </div>
  </section>
  <section>
    <div class="wrapper">
      <h2>Section 4</h2>
    </div>
  </section>
  <section>
    <div class="wrapper">
      <h2>Section 5</h2>
    </div>
  </section>
  <section>
    <div class="wrapper">
      <h2>Section 6</h2>
    </div>
  </section>
  <section>
    <div class="wrapper">
      <h2>Section 7</h2>
    </div>
  </section>
  <section>
    <div class="wrapper">
      <h2>Section 8</h2>
    </div>
  </section>
  <section>
    <div class="wrapper">
      <h2>Section 9</h2>
    </div>
  </section>
  <section>
    <div class="wrapper">
      <h2>Section 10</h2>
    </div>
  </section>
  <section>
    <div class="wrapper">
      <h2>Section 11</h2>
    </div>
  </section>
  <section>
    <div class="wrapper">
      <h2>Section 12</h2>
    </div>
  </section>
  <section>
    <div class="wrapper">
      <h2>Section 13</h2>
    </div>
  </section>
  <section>
    <div class="wrapper">
      <h2>Section 14</h2>
    </div>
  </section>
  <section>
    <div class="wrapper">
      <h2>Section 15</h2>
    </div>
  </section>
  <section>
    <div class="wrapper">
      <h2>Section 16</h2>
    </div>
  </section>
</main>

* {
  box-sizing: border-box;
}

html {
  height: 100%;
}

/* body and main sizing */

body {
  display: flex;
  align-items: center;
  justify-content: center;
  height: inherit;
}

main {
  gap: 50px;
}

main {
  overflow: scroll;
  scroll-snap-type: y mandatory;
  height: 450px;
  width: 250px;
  border: 3px solid black;
}

section {
  font-family: Arial, Helvetica, sans-serif;
  width: 150px;
  height: 150px;
  margin: 50px auto;

  scroll-snap-align: center;
}

.wrapper {
  width: 100%;
  height: 100%;
  border-radius: 5px;
  background: #eee;
  box-shadow:
    inset 1px 1px 4px rgb(255 255 255 / 0.5),
    inset -1px -1px 4px rgb(0 0 0 / 0.5);

  display: flex;
  align-items: center;
  justify-content: center;

  transition:
    0.6s background,
    0.6s color;
}

h2 {
  font-size: 1rem;
  letter-spacing: 1px;
}

section {
  container-type: scroll-state;
  container-name: snap-container;
}

@container snap-container scroll-state(snapped: y) {
  .wrapper {
    background: purple;
    color: white;
  }
}

<article>
  <h1>スクロール状態コンテナークエリーのある張り付くリーダー</h1>
  <section>
    <header>
      <h2>この最初の節は興味深い</h2>

      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</p>
    </header>

    ...
  </section>

  <section>
    <header>
      <h2>これはそれほどではありません</h2>

      <p>Confecta res esset.</p>
    </header>

    ...
  </section>

  ...
</article>

<article>
  <h1>スクロール状態コンテナークエリーのある張り付くリーダー</h1>
  <section>
    <header>
      <h2>この最初の節は興味深い</h2>

      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</p>
    </header>

    <p>
      Id deserunt exercitation in incididunt in dolor ipsum enim occaecat quis
      officia et. In dolore Lorem anim enim. Dolor nulla deserunt laboris irure
      incididunt labore tempor amet consectetur. Commodo eu tempor qui laboris
      duis ipsum adipisicing sunt. Do anim laboris commodo aliquip consequat ut
      eiusmod mollit sint nostrud sunt exercitation do.
    </p>

    <p>
      <strong>Est proident voluptate dolore.</strong> Aute irure ea culpa mollit
      <em>anim aute proident consectetur.</em> Duis deserunt consequat elit non.
      Cupidatat et amet anim labore est. Incididunt aute incididunt ex amet
      sint.
    </p>

    <img src="#" alt="Placeholder" />

    <p>
      Ullamco duis qui amet sunt eu nulla deserunt dolore officia irure nulla.
      <a href="#">Qui enim dolore est virtus;</a>
      Erat enim consectetur.
    </p>

    <p>
      Dolor aliquip officia id esse Lorem ullamco ea veniam.
      <strong>Voluptate cillum commodo anim veniam;</strong>
      Ad nulla amet anim eu incididunt reprehenderit proident in aliqua sit
      commodo magna.
    </p>
  </section>
  <section>
    <header>
      <h2>これはそれほどではありません</h2>

      <p>Confecta res esset.</p>
    </header>

    <p>
      <strong>Nam quid possums facere delius?</strong> Tu quidem reddes; Omnis
      peccant para dicts.
    </p>

    <p>
      <em>Pariatur ex cupidatat exercitation</em> reprehenderit quis culpa est
      proident pariatur ut.
    </p>

    <img src="#" alt="Placeholder" />

    <p>
      Non nulla pariatur cillum pariatur.
      <a href="#">Ipsum magna proident fugiat qui duis et.</a>
      Reprehenderit sit ipsum ut adipisicing aliqua eiusmod nulla consectetur
      deserunt dolor dolor.
    </p>

    <p>
      Cupidatat deserunt adipisicing adipisicing occaecat voluptate duis laboris
      excepteur labore et in aliquip aute. Quis aliquip reprehenderit dolor
      nostrud cupidatat nostrud nisi occaecat elit. Magna dolor ullamco eiusmod
      deserunt elit cillum qui amet. Dolor ad consequat anim commodo eu sunt do
      tempor eu velit est.
    </p>
  </section>
  <section>
    <header>
      <h2>この節で、多少なりとも不明点が解消されたでしょうか？</h2>

      <p>Dolor consectetur esse aute eiusmod ex reprehenderit cupidatat.</p>
    </header>

    <p>
      <strong>Ipsum quis sint incididunt aliquip</strong>. Duis tempor sunt enim
      Lorem ut officia cillum magna.
    </p>

    <img src="#" alt="Placeholder" />

    <p>
      <a href="#">Sint modo partes vitae beatae.</a>
      Ipsum esse nostrud incididunt ipsum ipsum esse aliquip Lorem enim.
      <strong>Sed hoc sane concedays.</strong> Voluptate qui consequat dolor
      dolor dolore ex. Enim dolor enim sit proident. Ad laborum incididunt
      dolore aliquip exercitation exercitation. Sunt cupidatat esse enim duis
      proident.
    </p>

    <p>
      Labore dolor cupidatat ut velit in minim cupidatat enim nisi proident
      eiusmod.
      <strong
        >Non nostrud consectetur est occaecat ut esse esse tempor eiusmod
        non.</strong
      >
      Cupidatat aliqua dolore est sit ad exercitation nostrud nostrud eu. Qui
      consequat et aliquip ea labore.
    </p>

    <p>
      Pariatur quis quis nostrud exercitation pariatur magna veniam Lorem
      commodo cupidatat irure reprehenderit. Nostrud pariatur est pariatur elit
      sit ea voluptate minim mollit. Aliqua pariatur cupidatat et cillum. Velit
      exercitation adipisicing laboris ullamco. Incididunt id eiusmod id cillum
      qui anim. Aliquip minim nostrud excepteur dolore deserunt pariatur eu
      eiusmod reprehenderit ullamco magna.
    </p>
  </section>
  <section>
    <header>
      <h2>これらのまとめ</h2>

      <p>Irure adipisicing occaecat dolore in ex voluptate.</p>
    </header>

    <img src="#" alt="Placeholder" />

    <p>
      <strong>An quidem modi?</strong>
      <strong
        >Excepteur non Lorem ex aliqua est reprehenderit incididunt sunt in
        ullamco in et;</strong
      >
      Aliqua veniam nulla magna anim labore amet proident nisi mollit non.
      Consequat incididunt exercitation nisi consequat culpa officia aliqua
      veniam. Anim aliqua consectetur pariatur mollit proident incididunt id.
    </p>

    <p>
      Non labore, inquit, de nomin. <em>Fall igniter possimus.</em>
      <a href="#">Preterits, inquit, gaud.</a>
      Est commodo adipisicing qui duis excepteur sit ea irure amet voluptate
      deserunt anim deserunt.
    </p>

    <p>
      Ad mollit laboris reprehenderit magna tempor. Consequat pariatur esse
      ipsum duis laboris.
    </p>
  </section>
</article>

/* 全般的なスタイル */

* {
  box-sizing: border-box;
}

html {
  font-family: Arial, Helvetica, sans-serif;
  height: 100%;
}

body {
  height: inherit;
  width: 100%;
  max-width: 540px;
  padding: 20px;
  margin: 0 auto;
}

p {
  line-height: 1.5;
}

img {
  display: block;
  width: 90%;
  margin: 30px auto;
  padding: 20px;
  border: 2px solid gray;
  aspect-ratio: 3/2;
}

header {
  background: white;
  position: sticky;
  top: 0;
  container-type: scroll-state;
  container-name: sticky-heading;
}

h2,
header p {
  margin: 0;
  transition: 0.4s background;
}

h2 {
  padding: 20px 5px;
  margin-bottom: 10px;
}

header p {
  font-style: italic;
  padding: 10px 5px;
}

@container sticky-heading scroll-state(stuck: top) {
  h2,
  p {
    background: #ccc;
    box-shadow: 0 5px 2px #0007;
  }
}