When it comes to the login and registration there are server side validations that are in place to ensure the values meet the minimum requirements. On the login page you can see the validations on lines 23-25:
$WAFV_Errors .= WAValidateEL(((isset($_POST["userpassword"]))?$_POST["userpassword"]:"") . "",6,50,true,2);
$WAFV_Errors .= WAValidateRX(((isset($_POST["userpassword"]))?$_POST["userpassword"]:"") . "","/[0-9]/",true,3);
$WAFV_Errors .= WAValidateRX(((isset($_POST["userpassword"]))?$_POST["userpassword"]:"") . "","/[a-z,A-Z]/",true,4);
You will see almost the exact same server validations on the users_Registration and users_Registration_full pages. The first one here is for the entry length, the second two are form the alphanumeric characters requirements. If you have the Validation Toolkit or CSS Form Builder you can edit these validations from the server behaviors area. You can also remove any of the validations that you do not want occurring.