While indeed neither markdown, much less JSON syntax has been intended as an SGML app, that doesn't stop SGML from parsing JSON, markdown, and other custom Wiki syntax using SHORTREF [1] ;) In fact, the original markdown language is specified as a mapping to HTML angle-bracket markup (with HTML also an SGML vocabulary), and thus it's quite natural to express that mapping using SGML SHORTREF, even though only a subset can be expressed.
