HTML Code

<!DOCTYPE html>
<html>
<head>
  <title>CSS :only-child Card Example</title>
  <style>
    body {
      font-family: system-ui, sans-serif;
      padding: 1.5rem;
      max-width: 24rem;
    }

    .card {
      display: flex;
      justify-content: center;
      align-items: center;
      min-height: 6rem;
      padding: 1rem;
      border: 1px solid #e2e8f0;
      border-radius: 0.5rem;
      background: #f8fafc;
    }

    .card > p:only-child {
      margin: 0;
      text-align: center;
      color: #1d4ed8;
      font-size: 0.9375rem;
      font-weight: 600;
    }
  </style>
</head>
<body>
  <h1>Notice</h1>
  <div class="card">
    <p>Standalone card message</p>
  </div>
</body>
</html>

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CSS :only-child - Centered Card Content