Classes for generation of not only (X)HTML code. It is possible to create also XML, XSL, SVG and many other - but possibilities are limited by original purpose: creation of HTML code.
SGML2KSS transforms one SGML document into a XML document that contains the content of the SGML document as well as markup information about the SGML instance such as OmitTag and ShortTag.
An implementation of DSSSL, a style language to format SGML or XML documents.
OpenJade contains backends for various formats (RTF, HTML, TeX, MIF, SGML2SGML, and FOT). This project also maintains OpenSP.