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HTML 

              
                <head>
  <meta name="viewport" content="width=device-width,initial-scale=1.0">
  <!--==============レイアウトを制御する独自のCSSを読み込み===============-->
  <link rel="stylesheet" type="text/css" href="https://assets.codepen.io/6329135/reset.css">
</head>

<body>
  <header id="header">
    <h1>WEB先案内の名言コレクション</h1>
  </header>
  <p class="caution">スクロールするとタイムラインの縦線が伸びる<br>↓ ↓ ↓</p>
  <main id="container">
    <section>

      <h2>MARVEL上映 と 世界の主な出来事</h2>

      <ul class="timeline">
        <li>
          <dl>
            <dt>2008</dt>
            <dd><strong>アイアンマン</strong>米大統領選オバマ当選</dd>
          </dl>
          <span class="border-line"></span>
        </li>
        <li>
          <dl>
            <dt>2009</dt>
            <dd><strong>アイアンマン２</strong>政権交代：民主</dd>
          </dl>
          <span class="border-line"></span>
        </li>
        <li>
          <dl>
            <dt>2010</dt>
            <dd><strong>ハルク</strong>JAL経営破綻</dd>
          </dl>
          <span class="border-line"></span>
        </li>
        <li>
          <dl>
            <dt>2011</dt>
            <dd><strong>キャプテン・アメリカ</strong>東日本大震災</dd>
          </dl>
          <span class="border-line"></span>
        </li>
        <li>
          <dl>
            <dt>2012</dt>
            <dd><strong>アベンジャーズ</strong>政権交代：自民</dd>
          </dl>
          <span class="border-line"></span>
        </li>
        <li>
          <dl>
            <dt>2013</dt>
            <dd><strong>マイティ・ソー/ダーク・ワールド</strong>スノーデン国家機密暴露</dd>
          </dl>
          <span class="border-line"></span>
        </li>
        <li>
          <dl>
            <dt>2014</dt>
            <dd><strong>ガーディアンズ・オブ・ギャラクシー</strong>STAP問題（マスゴミの集団リンチ）</dd>
          </dl>
          <span class="border-line"></span>
        </li>
        <li>
          <dl>
            <dt>2015</dt>
            <dd><strong>アベンジャーズ/エイジ・オブ・ウルトロン</strong>安全保障関連法成立</dd>
          </dl>
          <span class="border-line"></span>
        </li>
        <li>
          <dl>
            <dt>2016</dt>
            <dd><strong>シビル・ウォー/キャプテン・アメリカ</strong>米大統領選トランプ当選</dd>
          </dl>
          <span class="border-line"></span>
        </li>
        <li>
          <dl>
            <dt>2017</dt>
            <dd><strong>マイティ・ソー/バトルロイヤル</strong>森友・加計・日報問題</dd>
          </dl>
          <span class="border-line"></span>
        </li>
        </li>
        <li>
          <dl>
            <dt>2018</dt>
            <dd><strong>アベンジャーズ/インフィニティ・ウォー</strong>ゴーン元日産
              会長逮捕</dd>
          </dl>
          <span class="border-line"></span>
        </li>
        </li>
        <li>
          <dl>
            <dt>2020</dt>
            <dd><strong>アベンジャーズ/エンドゲーム</strong>COVID-19流行</dd>
          </dl>
          <span class="border-line"></span>
        </li>
      </ul>

      <h2>名言 1</h2>
      <p>人はそれぞれ事情をかかえ、平然と生きている</p>
      <p>ベストを尽くして失敗したら、ベストを尽くしたってことさ</p>
      <p>いちばんいけないのはじぶんなんかだめだと思いこむことだよ</p>
      <p>この世に生を受けたこと。それ自体が最大のチャンスではないか</p>
      <!--/area1-->
    </section>
    <section>
      <h2>名言 2</h2>
      <p>ろくな晩じゃねぇや。寝ちまえ、寝ちまえ。寝て起きりゃ別の日だ</p>
      <p>世の中ってオレより頭のいい人のほうが多いんだ</p>
      <p>何かを捨てないと前に進めない</p>
      <p>弱気は最大の敵</p>
      <!--/area2-->
    </section>
    <section id="area-3">
      <h2>名言 3</h2>
      <p>いつか、必ず、チャンスの順番が来ると信じなさい</p>
      <p>前向きにもがき苦しむ経験は、すぐに結果に結びつかなくても、必ず自分の生きる力になっていく</p>
      <p>「ゴールは遠いなぁ」と、がっかりするのも道のりです</p>
      <p>「負けたことがある」というのが　いつか　大きな財産になる</p>
      <!--/area3-->
    </section>
    <!--/main-->
  </main>

  <footer id="footer">
    <small>&copy; copyright.</small>
  </footer>

  <script src="https://code.jquery.com/jquery-3.4.1.min.js" integrity="sha256-CSXorXvZcTkaix6Yvo6HppcZGetbYMGWSFlBw8HfCJo=" crossorigin="anonymous"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/scrollgress/2.0.0/scrollgress.min.js"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-easing/1.4.1/jquery.easing.min.js"></script>
</body>
!

CSS 

              
                @charset "utf-8";

/*========= バー表示のためのCSS ===============*/

.scrollgress {
  z-index: 3; /*他のposition指定しているエリアより前に出すためz-indexの数字を大きく*/
  top: 70px !important; /* fixedで設置しているheaderの高さよりも下にバーを出す*/
}

/*========= バー表示のためのCSS ===============*/

/*タイムライン全体の設定*/
.timeline {
  max-width: 400px;
  width: 100%;
  margin: 50px auto;
  padding: 0 20px;
}

.timeline li {
  /*線の起点とするためrelativeを設定*/
  position: relative;
  list-style: none;
  padding: 0 0 20px 0;
}

.timeline dl {
  margin: 0 0 20px 3em;
}

.timeline dd strong {
  display: block;
  padding: 10px 0;
}

/*絶対配置で線を設定*/
.border-line {
  /*線の位置*/
  position: absolute;
  left: 0.2em;
  top: 0;
  width: 4px; /*線の太さ*/
  height: 0; /*はじめは高さを0に*/
  background: #40e0d0;
}

/*タイムラインの見出し横の丸の位置と形状*/
.timeline li::after {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  width: 10px;
  height: 10px;
  background: #aaa;
  border-radius: 50%;
}

/*========= レイアウトのためのCSS ===============*/

body {
  width: 100%;
  height: 100vh;
  vertical-align: middle;
}

h1 {
  font-size: 1.2rem;
}

#container h2 {
  font-size: 1.2rem;
  text-align: center;
}

#container p {
  margin-top: 20px;
}

small {
  background: #333;
  color: #fff;
  display: block;
  text-align: center;
  padding: 20px;
}

#header {
  position: fixed; /*Header固定*/
  top: 0;
  height: 70px; /*高さ指定*/
  z-index: 2;
  width: 100%;
  /*装飾のためのCSS*/
  background: #333;
  color: #fff;
  text-align: center;
  display: flex;
  justify-content: center;
  align-items: center;
}

section {
  padding: 50px 30px;
}

section:nth-child(2n) {
  background-repeat: no-repeat;
  background-position: center;
  background-size: cover;

  background-image: linear-gradient(
    0deg,
    rgba(255, 255, 255, 1) 5%,
    rgba(252, 225, 208, 1) 15%,
    rgba(255, 173, 214, 1) 50%,
    rgba(162, 186, 245, 0.8) 80%,
    rgba(255, 255, 255, 1) 95%
  );
}

#footer {
  position: relative;
  z-index: 2;
}

.caution {
  color: red;
  text-align: center;
  font-size: clamp(12px, 2.5vw, 14px);
  font-weight: bold;
  white-space: nowrap;
  margin-top: 100px;
}
!

JS 

              
                //線が伸びるための設定を関数でまとめる
function ScrollTimelineAnime() {
  $(".timeline li").each(function () {
    // それぞれのli要素の
    var elemPos = $(this).offset().top; // 上からの高さ取得
    var scroll = $(window).scrollTop(); // スクロール値取得
    var windowHeight = $(window).height(); // windowの高さ取得
    var startPoint = 100; //線をスタートさせる位置を指定※レイアウトによって調整してください
    if (scroll >= elemPos - windowHeight - startPoint) {
      var H = $(this).outerHeight(true); //liの余白と高さを含めた数値を取得
      //スクロール値から要素までの高さを引いた値を、liの高さの半分のパーセントで出す
      var percent = ((scroll + startPoint - elemPos) / (H / 2)) * 100; //liの余白と高さの半分で線を100％に伸ばす

      // 100% を超えたらずっと100%を入れ続ける
      if (percent > 100) {
        percent = 100;
      }
      // ボーダーの長さをセット
      $(this)
        .children(".border-line")
        .css({
          height: percent + "%" //CSSでパーセント指定
        });
    }
  });
}

// 画面をスクロールをしたら動かしたい場合の記述
$(window).on("scroll", function () {
  ScrollTimelineAnime(); // 線が伸びる関数を呼ぶ
});

// ページが読み込まれたらすぐに動かしたい場合の記述
$(window).on("load", function () {
  ScrollTimelineAnime(); // 線が伸びる関数を呼ぶ
});
!
999px

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