Just out of curiosity, I've been looking around and haven't found anything related.
HTML is a text markup language, and there are others like XML which is the only one with a clear explanation that I've found, but I've never seen/didn't find other things that can use other markup languages.
Is there another markup language that is widely used? If not, why isn't there another language that is so widespread? If yes, what are the main languages? Is there anything that in theory could be better than HTML?
There are several other markup languages that are used in environments and contexts other than the web.
There are some alternatives to HTML, but in the end they will be "transpiled" to HTML, after all, the browser does not know how to interpret these other languages. An example is the Haml language. Being better or worse is a matter of taste.