1. A method in a map server for providing data to a client device for rendering map images at the client device, wherein the client device is coupled to the map server via a communication network, the method comprising: receiving, from the client device, a request for map data for generating a first map image at the client device, including receiving a selection of a geographic region and a first map type; in response to the request for map data, (i) generating a plurality of vector descriptors, wherein each vector descriptor indicates a geometry of a respective map element in accordance with a vector graphics format, (ii) generating, for each of the plurality of vector descriptors, a respective style identifier that identifies a style according to which the corresponding map element is rendered, (iii) causing map data that includes the plurality of vector descriptors and the style identifiers to be transmitted to the client device; providing a description of first visual characteristics of the identified styles for the first map type to the client device, wherein the client device applies the first visual characteristics to the plurality of vector descriptors to generate the first map image; subsequently to receiving the request for map data, receiving, from the client device, a request for style data for generating a second map image corresponding to the previously selected geographic region and a second map type; and in response to receiving the request for the style data, (i) generating a description of second visual characteristics for one or more of the identified styles for the second map type, and (ii) causing the description of second visual characteristics to be transmitted to the client device, wherein the client device applies the second visual characteristics to the plurality of vector descriptors at the client device to generate the second map image.