Well screw Oxford, Cambridge, and Harvard.
How about the possessive apostrophe “s” for names or words ending in the letter “s”? Who said it was proper to write “Williams’s”, as in belonging to the Williams family? The proper way is Williams’. No second “s”. To me, as someone who has a last name ending in “s”, the former will never be correct. The nuns made sure I remembered that one, and God bless them. Perhaps elementary schools should go back to diagramming sentences.