In the 1954 film Sabrina, Humphrey Bogart and William Holden portray two brothers from a wealthy family. One is a nose-to-the-grindstone businessman, the other a feckless playboy. A few subtle details, such as the angle of Bogart's hat brim, help convey their relative roles and character to the audience. These cues were clear enough to the film's original viewers, and still reasonably understandable.
For a film set 600 years ago, how practical is it to convey similar cues to a modern audience? A few medievalists might know that one style of sleeve denotes a respectable lady, and another that the wearer is a prostitute, but only a tiny minority of a modern audience could be expected to know this sort of detail.
If this information isn't conveyed then the modern audience is missing some important information about the relationship between the characters. What to do?
Ian McKellen's 1995 Richard III represents one solution. Setting the medieval story in a 1930s alternate reality transposes it into a society more comprehensible to modern viewers. The Woodvilles are portrayed as Americans, capturing their status as isolated outsiders in a way that the Olivier version missed. The interfamilial horror of Richard's coup is also underlined and the social relationship between Richard and his henchmen made more accessible.
But transposing a story to another era also has a downside. Shakespeare's Richard III may have manipulated popular opinion to gain power, but he wasn't a fascist. Fitting him into a fascist template is false to the actual Richard III, who belonged to a different era and thought differently.
The director of Knight's Tale followed a related strategy. One of the key plot elements is a romance between a rising commoner who is passing himself off as a knight and Jocelyn, an aristocratic lady. How can the director make the social gulf between them as clear to the audience as it would be to the hero? How can the director convey, not merely "she has expensive clothes" but "one of her dresses costs enough to support an ordinary family for a year?" In the film, Jocelyn wears clothes that evoke a modern couture wearing jetsetter. In one scene she wears a hat that might remind the viewer of one worn by Audrey Hepburn. In another she wears a strategically sheer dress that has more in common with modern high fashion than the Middle Ages. Her hair is the sort of expensive confection that can only be achieved with professional help, and perhaps not at all before the invention of styling mousse.
What she wears isn't necessarily much like what a woman of her rank would actually have worn in the 14th century. But it does convey a sense of expensively conspicuous consumption to the typical modern audience, and probably does a better job of doing so than literally correct costume and hairstyles would.