HTML Code

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

    label {
      display: block;
      margin: 0 0 6px;
      font-weight: 700;
    }

    input {
      padding: 8px 10px;
      font: inherit;
    }

    input:invalid {
      outline: 2px solid #dc2626;
      outline-offset: 2px;
    }
  </style>
</head>
<body>
  <label for="code">Code (a-z only)</label>
  <input id="code" type="text" pattern="[a-z]+" value="Test1">
</body>
</html>

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CSS :invalid Pseudo-class — pattern