To give you an idea of what SGML is capable of, see my tutorial at [1]. It implements a lightweight content app where markdown syntax is parsed and transformed into HTML via SGML short references, then gets HTML5 sectioning elements inferred (eg. the HTML5 outlining algorithm is implemented in SGML), then gets rendered as a page with a table of content nav-list linking to full body text, and with HTML boilerplate added, all without procedural code.

[1]: http://sgmljs.net/docs/producing-html-tutorial/producing-htm...