Sentence:  It is obvious that the reasonable principles do not require a merchant ship to bear a green light at her starboardside and a red light at her <WSD>portside</WSD>, but if each State were to regulate the colour and the place of the lights with unlimited sovereignty, and without regard for the uniformity, the regulation would be a danger instead of a measure of precaution. 
Meaning of the word marked with <WSD> tags:  The port (left) side of a boat.