Cascading Stylesheets (CSS) are a mechanism for attaching style information (font, weight, size, color, and so forth) to elements in an HTML document without abusing HTML to do so.
The FreeBSD DSSSL stylesheets include a reference to a stylesheet, docbook.css, which is expected to appear in the same directory as the HTML files. The project-wide CSS file is copied from doc/share/misc/docbook.css when documents are converted to HTML, and is installed automatically.