html/forms/form-validation/fruit-start.html

<!DOCTYPE html>
<html>

<head>
    <meta charset="utf-8">
    <title>Favorite fruit start</title>
    <style>
      input:invalid {
        border: 2px dashed red;
      }

      input:valid {
        border: 2px solid black;
      }
    </style>
</head>

<body>
    <form>
      <label for="choose">Would you prefer a banana or a cherry?</label>
      <input id="choose" name="i_like">
      <button>Submit</button>
    </form>
</body>

</html>