HTML Code

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

    input:required {
      background: #fef08a;
      border: 1px solid #ca8a04;
      padding: 0.4rem 0.6rem;
    }
  </style>
</head>
<body>
  <label>
    Name (required)
    <input type="text" name="name" required>
  </label>
</body>
</html>

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CSS :required — input