I believe, however, that a fourth layer of separation is being neglected: the data layer. This layer is represented by server side scripts that process and retrieve information from a data source. More often than not, we find this layer embedded messily into the structural layer. When the goal is to build modular architectures that are flexible and adaptable, combining structure and data processing is, in the long run, going to be very costly conceptual mistake. Through the use of a very promising XML technology, XSLT, we can free our data processing and retrieval logic from our display and structural logic completely and build web applications that are easier to understand and faster to iterate.
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