• Check and verify SGML tools
• Parse and validate mostly markup-less files with SGML tools
• Remember DTDs such as qwertz, linuxdoc, and DocBook 2
• Validating markdown with SGML tools?

See http://sgmljs.net/docs/sgmlrefman.html#sgml-declaration-for-markdown, http://esu.culintec.de/?q=node/601, etc. ;)

I my context, not enlighting:

• http://schematron.com/2018/02/common-markdown-specification-so-when-is-it-a-good-idea-to-throw-theory-out/