The HTML version of this document makes use of a custom CSS stylesheet for rendering HTML elements slightly differently (in fact, in this small example only some colours are changed). The CSS stylesheet is the section called “selfdocbookx.css”, and the DocBook HTML stylesheet is told about it by our custom XSL layer: the html.stylesheet parameter is set to the name of the CSS file we want to use. See the section called “css.xsl” for how that's done.