1. In a markup language processing device, a computer-implemented method in which a computer system performs security processing operations on markup language messages on behalf of a recipient computerized device that is to receive the markup language messages, the method comprising: receiving a message from a client computerized device including portions of tagged data formatted in a markup language; applying a transform selection rule set to at least one tagged rule selection data portion in the message to select at least one markup language transformation to apply to at least one tagged pre-transform data portion within the message; applying the selected at least one markup language transformation to the at least one tagged pre-transform data portion to transform the at least one tagged pre-transform data portion in the message to at least one tagged post-transform data portion in the message according to at least one transformation function associated with the at least one markup language transformation; and conditionally forwarding the message back to the client computerized device including the at least one tagged post-transform data portion to the recipient computerized device such that the recipient computerized device is not required to process the message due to operation of the at least one markup language transformation within the markup language processing device.