Standard system of standard generalized markup language.

The first level is the metalanguage standard-"universal standard"; The second level is the basic standards, such as: document style semantics and standardized language standard (foreign acronym: DSSSL), which are the basic standards of this system; The third level is specific application standards, such as hypertext markup language and other standards. That is to say, before the emergence of the Extensible Markup Language standard, the "generalized markup language" standard has formed a perfect system, and there are many application systems and application standards based on it. As shown in the figure below.