cup.html

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <link rel="stylesheet" href="normalize.css">
  <link rel="stylesheet" href="./cup/style.css">
  <title>Bulb</title>
</head>

<body>
  <div class="cup">
    <div class="top">
      <div class="tea"></div>
    </div>
    <div class="handle"></div>
    <div id="vapours" class="vapours">
    </div>
  </div>
  <script>
    const v = document.getElementById("vapours");
    const f = document.createDocumentFragment();
    for (let i = 0; i < 30; i++) {
      let d = document.createElement('div');
      d.classList.add("vapour");
      d.style.setProperty("--i", Math.floor(Math.random() * 10));
      f.appendChild(d);
    }
    v.appendChild(f);
  </script>

</body>

</html>