HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <title>CSS % and em Example</title>
  <style>
    body {
      margin: 0;
      padding: 1.5rem;
      font-family: system-ui, sans-serif;
      background: #f1f5f9;
    }

    .container {
      width: 80%;
      max-width: 480px;
      padding: 2em;
      background: #fff;
      border: 1px solid #e2e8f0;
      border-radius: 0.5rem;
      font-size: 16px;
    }

    .container p {
      margin: 0;
      color: #334155;
      font-size: 1em;
    }
  </style>
</head>
<body>
  <div class="container">
    <p>Container is 80% of parent width. Padding is 2em (32px when font-size is 16px).</p>
  </div>
</body>
</html>

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Relative Length — % and em