1. In a server computer system that provides Web pages to requesting client computer systems, the Web pages potentially including content from locations that are external to the server computer system, the server computer system including a cache that stores portions of cached content previously received from locations external the server computer system, a method for causing a cache entry to be dependent on a customized dependency, the method comprising the following: an act of accessing an extensible cache dependency base class; an act of deriving a customized cache dependency class from the extensible cache dependency base class, the customized cached dependency class for implementing the customized dependency; an act of accessing a portion of content that is to be delivered to a client computer system; an act of creating a cache entry that associates the customized dependency with the accessed portion of content; and an act of inserting the cache entry into cache such that the validity of the cache entry is dependent on the customized dependency.