I found this additional class that can be added to the checkout form that will cause an alert window to be displayed when an input does not validate. It may be useful for forms where the light red background is not obvious to the user why they can’t submit the form to checkout.
This can be especially useful for the State/Province menu selection, since css cannot highlight them in the same red background in some browsers, and only the field label is highlighted.
To add this option, just add the class “validation-alerts” to the form in the file checkout.php (will require enabling theme content templates).
<form action="<?php shopp('checkout','url'); ?>" method="post" class="shopp validate validation-alerts" id="checkout">