In /css/section-cart.css you need to change line 186 from
table.col-one label .khxc_formfield {
to
table.col-one label .khxc_formfield, table.col-one label .khxc_formfield_rnc {
and line 193 from
* html table.col-one label .khxc_formfield {
to
* html table.col-one label .khxc_formfield, * html table.col-one label .khxc_formfield_rnc {

This is to make sure the form doesn’t fall apart when a requested field is missing.

Now to highlight the missing fields you should add the following lines to the same file, preferably on line 196 and add styles that will highlight the fields:
table.col-one label .khxc_formfield_rnc {
border: 1px solid #107B84;
padding: 1px;
}

The example will turn the border link green. Done and tested in Firefox on a Vista machine.

What I suggest is that you at least make the select field highlighted – that way when scanning the form I’d see what’s missing. It’s not the ultimate solution but it’s better than what you have now.

In any case, we’ll take another look at this problem (though it’s the first time it’s come up–at least publicly!). Thanks for noting it, and despite our own form design issues, I hope you enjoy Luke’s book!