These are DSSSL stylesheets for the DocBook DTD (they are also completely compatible with the DocBook XML DTD). DSSSL is a stylesheet language for both print and online rendering. The acronym stands for Document Style Semantics and Specification Language. It is defined by ISO/IEC 10179:1996. For more general information about DSSSL, see the DSSSL Page at http://www.jclark.com/dsssl