If one tries to add an item and mandatory or important select field isn't filled, field isn't highlighted like input fields.
As far as I can see, only the select2 elements are not highlighted after the page reloads when mandatory fields have not been filled. I couldn't locate how the "missing" properties to the elements' class are generated (like <input type="text" id="tag_952_subfield_y_371260" name="items.itype" class="input_marceditor items.itype noEnterSubmit missing" maxlength="9999" value="">), but if there was a check on whether the element is a pulldown list/any other type of element not getting highlighted, their styling could be processed separately.