You seemed unclear on what SGML is, so here's a quick summary:

SGML, to tell a simplifying lie, is the idea of using tag names enclosed in less-than and greater-than characters to mark up text. The original DocBook is one application, HTML is another.

XML is a further evolution of SGML, which both constrains and extends SGML to enable new ways of defining and working with applications of the format.

The counterpart of XML Schema in SGML is the DTD, the Document Type Definition, and the counterpart of XSL is DSSSL, which is a form of Scheme (yay!).