Complex sentence: Diving masks may have a single, durable, tempered glass plate, or two lenses in front of the eyes and a "skirt" of synthetic rubber or silicone elastomer to create a watertight seal with the diver's face.
Split into simpler sentences: Diving masks may have a single, durable, tempered glass faceplate, or two lenses in front of the eyes. There is usually a "skirt" of synthetic rubber or silicone elastomer to support the frame and lenses and create a watertight seal with the diver's face.