V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

DSSSL
       [standard] Document Style Semantics and Specification Language
(DTD, ISO, IEC, DIS 10179)


The Free On-line Dictionary of Computing (30 December 2018):

Document Style Semantics and Specification Language
DSSSL

   (DSSSL) An ISO standard under preparation, addressing the
   semantics of high-quality composition in a manner
   independent of particular formatting systems or processes.
   DSSSL is intended as a complementary standard to SGML for
   the specification of semantics.