Note that in the specific case of XML-RPC, extensibility is forbidden by the spec: A <fault> struct may not contain members other than those specified. Thi

Note that in the specific case of XML-RPC, extensibility is forbidden by the spec: A <fault> struct may not contain members other than those specified. This is true for all other structures. We believe the specification is flexible enough so that all reasonable data-transfer needs can be accomodated within the specified structures. If you believe strongly that this is not true, please post a message on the discussion group. At one point the author did intend to allow elements to contain user-defined children[0], but it seems like their position changed between that mailing list post and the spec update 7 days later. [0] https://web.archive.org/web/19991010205056/http://discuss.userland.com/msgReader$2135 https://web.archive.org/web/19991010205056/http://discuss.us...