El patrón HTML no funciona

1.
<input type="text" name="formField" pattern="[a-zA-Z0-9]{6}" required>

2.
<form>
    <input type="text" pattern="[a-zA-Z0-9]{6}" name="formName" />
    <input type="submit" value="OK" />
</form>

3.
<html>
<body>    
<form action="demo_form.asp">
  Country code: <input type="text" name="country_code" pattern="[A-Za-z]{3}" title="Three letter country code">
  <input type="submit">
</form>    
</body>
</html>

4.
SECRET MYSTERY