Thanks for asking!
Here's the form in question
If you put an item in the shopping cart and hit checkout you'll see the form in question.
If you try leaving fields blank on the form, you will see that they turn pink when you try to checkout. I can't figure out how to get the dropdown fields (State or Province) to do the same. I also don't know how to get a message to show up that alerts the user to the errors in the form (though I know that latter thing is not a CSS issue).
I built this with Wordpress and the Shopp plugin and am not sure how to access the HTML (only the parsed HTML via Firebug) but this an example of what I've got when I try submitting a blank form...
Code:
<div class="left">
<input id="shipping-city" class="required disabled" type="text" value="12 Bolduk Lane" title="City shipping address" name="shipping[city]" disabled="disabled">
<label for="shipping-city">City</label>
</div>
<div class="right">
<select id="shipping-state-menu" class="required shipping-state disabled" title="State/Provice/Region shipping address" name="shipping[state]" style="display: inline;" disabled="disabled">
<input id="shipping-state" class="required shipping-state disabled" type="text" value="CT" title="State/Provice/Region shipping address" name="shipping[state]" disabled="disabled" style="display: none;">
<label for="shipping-state">State / Province</label>
</div>