According to Debrett's , the authority on British etiquette and manners since 1769, she did, but not for the reason we think. "A gray or black top hat is worn with morning dress for racing but at most other events it is carried rather than worn and may be dispensed with," the website details, meaning it's Delevingne's hat (not the fact that she wore a suit) that's the problem here. And despite the wedding invitation calling for women to arrive wearing a "day dress and hat," (according to Her, an Irish website ) the 26-year-old chose an Emporio Armani tuxedo complete with a cropped jacket, slim-fitting trousers, a cummerbund, a white button-up shirt, and a patterned gray tie. She finished the look with a pair of black stilettos.