The DocBook XSL stylesheets are a set of stylesheets for use with an XSLT engine (such as xsltproc or Saxon) for transforming DocBook XML documents into other DocBookOutputFormats, such as HTML, PDF, Microsoft HTML Help, and man pages. NormanWalsh designed the XSL stylesheets and continues to actively develop and maintain them, making bug fixes and feature enhancements to them and regularly releasing updated versions (with contributions from members of the DocBookOpenRepository, where the stylesheets are now maintained). Documentation for the XSL stylesheets is available on the DocBookXslStylesheetDocs page at this Wiki.
Build the Stylesheets from Source
If you plan to make modifications to the standard stylesheets (in order to contribute patches for example) you might want to test your changes. How to build the styleshetfrom sources is explained in DocBookXslBuild