csexp and XML differ in that csexp is a data-representation format, while XML includes a data-representation format and also a schema mechanism. Thus, XML can be "configured" for particular kinds of data, which conform to some grammar (say, HTML, ATOM, SVG, MathML, or new ones as needed). It has languages for defining document grammars: DTD is defined by the XML standard itself, while XSD, RelaxNG, and Schematron are commonly used with XML for additional features, and XML can also work with no schema. csexp data can of course be operated on by schemas implemented at a higher level, but provides no such mechanism itself.