HTML Code

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

    form {
      max-width: 20rem;
      padding: 12px;
      border: 2px solid #94a3b8;
    }

    form:invalid {
      border-color: #dc2626;
    }

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

    input {
      width: 100%;
      box-sizing: border-box;
      padding: 8px 10px;
      font: inherit;
    }
  </style>
</head>
<body>
  <form>
    <label for="email">Email</label>
    <input id="email" type="email" value="bad@@mail" required>
  </form>
</body>
</html>

Lines: 0 | Characters: 0

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CSS :invalid Pseudo-class — form:invalid