Particletree’s Ryan Campbell offers up the 4th layer of separation for web development. The predominant thinking has been three layers of separation: structural, presentational, and behavioral – completely ignoring the 4th layer: data. This is a very important article to read if you are a serious web developer. Through the use of XSLT, developers are now able to supply the data in any form they wish. Structured data makes web development simpler on the front end, since we no longer care how the data is stored on the backend, and XSLT takes care of the rest. Ryan also offers some links to more information on the subject, and illustrates how Google uses this approach in its AJAX applications.