I just came across an answer by a new user which looks very unusual to me: https://mathoverflow.net/a/139275/21095
Specifically, there is very little mathjax in this answer. Instead the vast majority of the math is pre-formatted text, including hard-coded bold and italic symbols, hard-coded subscripts and superscripts, etc. Please take a look at the code for the answer yourself, as I find it hard to describe.
Edit: According to the author of the post, it was composed using XCompose.
One problem is that the text does not even render in an alternative web browser I use called midori (not mainstream, but highly developed). I left a comment suggesting that the answer be rewritten using latex/mathjax code.
My questions are:
How and why does the software accept and recognize these symbols? Are all the symbols unicode?
Is this kind of code used elsewhere? Or is this a completely isolated case?
Should such code be replaced with mathjax?
Update: It would appear that all the symbols are indeed unicode. I am unable of checking this statement thoroughly, but it seems plausible from the answer by Rohan Lean below, and from a cursory look at a list of unicode characters, e.g. here.