Current release of docbook-xsl is 1.79.1 from Dec 2015. (Port currently has version 1.76.1 from 2010.)
See also https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=221094 which has a patch. Would be great to get that upgraded, as right now the PostgreSQL documentation can't be built the port (and no doubt other things too).
adding a depends since the dupe has info on a showstopper
*** This bug has been marked as a duplicate of bug 231628 ***