1. A method of transforming a source data in a source markup language to a target data in a target markup language according to a set of transformation rules, said set of transformation rules defining mappings of a plurality of source tags in said source markup language to a plurality of target tags in said target markup language, each of said plurality of source tags having an associated content in said source data, said method being implemented in a digital processing system, said method comprising: receiving said set of transformation rules; pre-processing said set of transformation rules to identify a plurality of referenced tags in said set of transformation rules, wherein each of said plurality of referenced tags is one of said plurality of source tags in said source markup language, wherein said set of transformation rules specify that the content of each of said plurality of reference tags is needed to apply at least one transformation rule of another source tag, wherein said set of transformation rules further specify that the transformation of said another source tag is performed after applying the transformation rules designed to generate the content of the corresponding reference tag when transforming said source data to said target data; storing said plurality of referenced tags identified by said pre-processing in a memory; retrieving sequentially each of said plurality of source tags from said source data after said pre-processing and said storing, and for each retrieved source tag: if the retrieved source tag is one of said plurality of referenced tags, storing the content of the retrieved source tag associated with said one of said plurality of referenced tags in said memory; if said plurality of transformation rules contain a transformation rule defining a corresponding mapping for the retrieved source tag, generating a portion of said target data based on said corresponding mapping defined in said transformation rule and the content of retrieved source tag, wherein said generating retrieves said content of a first referenced tag from said memory for use with said corresponding transformation rule in forming said portion if said corresponding transformation rule includes said first referenced tag.