HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <title>opacity: 0.5</title>
  <style>
    body {
      font-family: Arial, sans-serif;
      margin: 16px;
      color: #0f172a;
    }

    p {
      margin: 0 0 12px;
      max-width: 40rem;
      line-height: 1.5;
    }

    section {
      background: #e2e8f0;
      padding: 16px;
    }

    div {
      opacity: 0.5;
      background: #2563eb;
      color: #fff;
      padding: 16px;
      font-weight: 700;
    }
  </style>
</head>
<body>
  <p>The blue box uses <strong>opacity: 0.5</strong>. Fill and text both fade, so the gray parent shows through.</p>
  <section>
    <div>Half opaque</div>
  </section>
</body>
</html>

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CSS opacity Property — Semi-transparent Box