The pure XSLT implementation of CSS Within uses fn:transform to run the original transformation and post-processes the result to remove unwanted CSS Within elements. It then processes the stylesheet files themselves as XML documents to find all of the css:rule and css:media elements and construct stylesheets.

This approach does not work with separate compilation, as in that case the source code for the original transformation is no longer available. It also does not work with packages, since the package location mechanism is not visible at the XSLT level. However, it is still very useful.