While XML is a solid standard format for defining document content, it is not much fun for formatting. I have taken the xml format and created a php translator. This way I can implment my own custom tags.
This started as a way to define a standard template for sending emails containing form data. From there it has grown into a more robust web page api. Eventually I will get around to creating document describing current features and how it works. Until then I will just say that I have since expanded it to allow for true mashups. See the rule 1 calculator to see it in action.