There Are Very Specific Rules When It Comes to Dressing For the Royal Calendar

There is a VERY extensive list of rules, but basically it's:

• Women: Formal daywear, which means dresses and skirts of "modest length" (just above the knee or longer), and straps on dresses should be at least an inch wide. Fascinators are no longer allowed as headpieces need to have a base of four inches or more. No off-the-shoulder or halter tops, and ladies are advised to wear wedges over stilettos so they do not sink heel-first into the ground.
• Men: Black or gray morning dress, which means a tailcoat and waistcoat with matching trousers, along with a dress shirt and tie. No cravats. Black or gray top hat and black shoes.