An apparatus (10) comprises a mobile device (1) that communicates with a web server (2) over a mobile network (3). The capabilities of the mobile device (1) can be described by device attributes in a device database. An aggregation engine (5) is provided to compile an aggregated device database from a number of source device databases. Both the source device databases and the aggregated database are stored in a device attribute store (4). When a user of the mobile device (1) attempts to access a web resource hosted on the web server (2), the web server (2) looks up the values of device attributes in the aggregated device database before returning data to the mobile device (1) that is appropriate for its capabilities. The aggregated database includes information regarding the value of each device attribute and also information regarding the probability that this value is correct.