ComboBox mandatory field's border Color should be red

Example: https://paste.apache.org/s6vuu
Output : 

If there is nothing selected in ComboBox or the "Not selected" value is selected then the borderColor should be red in case of mandatory field

Start with the linked paste example and reproduce the ComboBox behavior for a mandatory field with no selection or the “Not selected” value. Locate the ComboBox validation and border-color handling, then verify that the border is red in both cases and remains correct when a valid value is selected.

Written by the indexing model from the issue text.