XML is rather successful outside of the web in spite of all the vitriol poured on it. And modern web is under a specific and unique combination of pressures to

XML is rather successful outside of the web in spite of all the vitriol poured on it. And modern web is under a specific and unique combination of pressures to serve as a reference for the rest of software development. If you looking for something that deserves the name of "defective" you don't need to go any further. For example, the problem of math is not only to render it in the browser. At the very least we may want to render it on the server and to index it. And since math expressions are in a document, there's a general need to process them programmatically for a variety of purposes, some of which are not even clear at the moment. With the KaTeX or MathJax solution each such scenario would have to include KaTeX or MathJax or a custom parser for the underlying TeXlike language the only upside of which is that it's somewhat well-known and more or less easy to write. With MathML these and other scenarios can be handled with the standard XML toolchain. (And this doesn't mean we need to exclude that neat TeXlike language if we need to input it: we only need to add a step that transforms it into MathML.) MathML, is, of course, not simple, but it addresses both presentational and semantic sides of a formula, something that no other solution does. It's complex because the math is complex.