Actually, in SGML (but not the XML subset of SGML) it's common to define char sequences (shortrefs) an SGML parser would replace into tags, which together with

Actually, in SGML (but not the XML subset of SGML) it's common to define char sequences (shortrefs) an SGML parser would replace into tags, which together with tag inference (also left out from XML) can make it possible to at least type casual and high school math naturally as ASCII math expressions and have those render into presentational MathML. Though that approach clearly has limitations (fraction style, roots, infix expressions, etc.). It's also conceivable to attempt to customize SGML's concrete syntax to recognize LaTeX (but not TeX proper) such that it treats "\bla{...}" as <bla>...</bla>, etc. But the fact that nobody had bothered probably tells us it's not all that useful.