1. A method for managing cached document fragments, the method comprising: storing a schema document that describes a class of markup language documents in terms of constraints on the structure and content of markup language documents in the class; wherein the schema document includes one or more caching policies; wherein the one or more caching polices are applicable only to fragments of markup language documents in the class; wherein at least one caching policy of the one or more caching policies indicates a duration for caching a fragment of a markup language document in the class; storing a plurality of markup language documents that, according to the schema document, are in the class; wherein each markup language document of the plurality of stored markup language documents has, according to the schema document, at least one fragment to which the at least one caching policy applies; and serving the plurality of markup language documents and the schema document to a client process which causes the client process to cache fragments of the plurality of markup language documents in accordance with the at least one caching policy; wherein the method is performed by one or more computing devices.