1. A method, comprising: receiving at a proxy server a request for a Web document, said request originating from a client application executing on a client device communicably coupled to the proxy server; retrieving, by the proxy server, a copy of the requested Web document, the proxy server modifying the requested Web document by inserting a set of computer-readable instructions which when executed by a processor of the client device on which the client application runs cause the client application to perform one or more activities associated with network traffic policy enforcement, including reporting, to the proxy server, information concerning the Web document; downloading, from the proxy server to the client application, a modified version of the requested Web document that includes the set of computer-readable instructions; receiving, at the proxy server and from the client application responsive to execution of the set of computer-readable instructions by the processor of the client device on which the client application runs, the information concerning the Web document, the proxy server using said information to determine whether the Web document is subject to an existing network traffic policy; and if the Web document is subject to the existing network traffic policy, enforcing the existing network traffic policy in connection with subsequent requests for the Web document; otherwise, not enforcing the existing network traffic policy in connection with subsequent requests for the Web document.