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 :valid and :invalid</title>
  <style>
    :valid {
      border: 2px solid #15803d;
    }

    :invalid {
      border: 2px solid #b91c1c;
    }
  </style>
</head>
<body>
  <form>
    <p><input type="email" required value="you@example.com" aria-label="Valid email"></p>
    <p><input type="email" required value="not-an-email" aria-label="Invalid email"></p>
  </form>
</body>
</html>

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CSS :valid - Valid and Invalid