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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>CSS font-size em Example</title>
  <style>
    body {
      font-family: system-ui, sans-serif;
      padding: 1.5rem;
      line-height: 1.6;
    }

    .card {
      font-size: 18px;
      max-width: 22rem;
      padding: 1rem 1.15rem;
      border: 1px solid #e2e8f0;
      border-radius: 0.5rem;
      background: #f8fafc;
      color: #334155;
    }

    .card button {
      display: block;
      margin-top: 0.65rem;
      font-size: 0.875em;
      padding: 0.4rem 0.85rem;
      border: 1px solid #cbd5e1;
      border-radius: 0.35rem;
      background: #fff;
      cursor: pointer;
    }
  </style>
</head>
<body>
  <h1>em Relative Sizing</h1>
  <div class="card">
    Card text is 18px.
    <button type="button">Button at 0.875em (~15.75px)</button>
  </div>
</body>
</html>

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CSS font-size Property - em Nesting