<!DOCTYPE html>
<html lang="ja">
<head>
  <meta charset="UTF-8">
  <link rel="stylesheet" href="popup.css">
</head>
<body>
  <section>
    タイトル
    <p id="page_title"></p>
    <button type="button" id="copy">コピー</button>
  </section>
  <script src="popup.js"></script>
</body>
</html>

<!DOCTYPE html>
<html lang="ja">
<head>
  <meta charset="UTF-8">
  <link rel="stylesheet" href="popup.css">
</head>
<body>
  <section>
    タイトル
    <p id="page_title"></p>
    <button type="button" id="copy">コピー</button>
  </section>

  <script src="popup.js"></script>
</body>
</html>

Split section and script in html
Split section and script in html
