As for the lack of libraries. Technically they are possible, but are not used that often. Maybe they are not that necessary. XML's idea is a federation of notations. Each notations is semantic, that is it makes only specific distinctions. XSLT transforms between two such notations. Since each combination is unique, there is little to reuse. Where other tools use libraries XSLT uses separate stylesheets and just rearranges them differently.