Anything described as de rigueur is required because of etiquette, fashion, or custom. De rigueur things are expected by society. Big floppy shoes and a red nose is de rigueur for a clown.
The literal meaning of this French loan word is “in strictness,” and it has to do with strictly following etiquette and manners. It is de rigueur to put your napkin in your lap in a restaurant. It is de rigueur to send a thank-you note after receiving a gift. It doesn’t always have to be so serious, a sideways ponytail is de rigueur in an 80’s aerobics class.