From Wikimedia.
Coiffures in the pre-Revolutionary era were wider than tall, requiring large hats, such as the diaphanous feathered example worn by Madame Élisabeth. She wears a dress with a deep scoop neckline fringed by a flounce where a bertha would go on an evening dress. The tent bodice has a set of large buttons running down the center of her bodice. It would be interesting to see what the rest of her dress looks like. There appears to be a modesty piece under her striped fichu.
Keywords: 1787, Labille-Guillard, Élisabeth Philippine de France, Élisabeth de Bourbon, Madame Élisabeth, Bourbon family, Princess, French, bouffant coiffure, feathered cap, high neckline, fichu, modesty piece, scoop neckline, tent bodice, flounces, close sleeves, buttons