All storefront windows, glass doors, or doors with glass openings, may have signs, shades, blinds, curtains, drapes, or other coverings on such glass doors, or doors with glass openings, provided that the area covered does not exceed twenty (20%) percent of the total window, glass door or door with a glass opening. All such windows, glass doors or doors with glass openings must remain at all times eighty (80%) percent unobstructed.