<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>JavaScript</title>
  <style>
    #subtitle {
      width: 300px;
      padding: 20px;
      border: 1px solid #FF0000;
    }
  </style>
  <script>
    function initiate() {
      const element = document.getElementById("subtitle");
      let width = element.clientWidth;
      width = width + 100;
      element.style.width = width + "px";
    }
  </script>
</head>
<body onload="initiate()">
  <section>
    <h1>Website</h1>
    <p id="subtitle">The best website ever!</p>
  </section>
</body>
</html>