HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>CSS optional dashed required solid</title>
  <style>
    input:optional {
      border: 2px dashed #64748b;
    }

    input:required {
      border: 2px solid #0f172a;
    }
  </style>
</head>
<body>
  <form>
    <p><label>Username * <input type="text" required></label></p>
    <p><label>Phone <input type="tel"></label></p>
  </form>
</body>
</html>

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CSS :optional - Dashed Optional Solid Required