Breaking all the previous “rules” of fashion, Victoria insisted on wearing white to her wedding, establishing the “rule” for that activity.. forever. Her dictates from this point on, told all Britains (and so all other countries who were following their lead) what was appropriate to wear on what occasion.
Note: this is just before photography became available. Her early reign is documented in portraits and sketches. Her late reign and her children would be well documented, especially since her daughter Beatrice would become a photographer and have her own dark room.
(In 1840, Queen Victoria was married to Prince Albert)