SGML
From Just Solve the File Format Problem
SGML (Standard Generalized Markup Language) is a general format for markup languages descended from a 1960s IBM effort ("Generalized Markup Language"). It uses Document Type Definitions (DTDs) to define the specifics of a particular markup format. HTML was originally developed as a version of SGML, but has drifted away from this and the current HTML 5 is explicitly no longer based on SGML. XML is a somewhat simplified variant of SGML.
