/usr/share/sgml : SGML data (optional)

/usr/share/sgml contains architecture-independent files used by SGML applications, such as ordinary catalogs (not the centralized ones, see /etc/sgml), DTDs, entities, or style sheets.

The following directories, or symbolic links to directories, must be in /usr/share/sgml, if the corresponding subsystem is installed:

Directory Description
docbook docbook DTD (optional)
tei tei DTD (optional)
html html DTD (optional)
mathml mathml DTD (optional)

Other files that are not specific to a given DTD may reside in their own subdirectory.