In Brilliant.org, when we look at the source of the pages, we see that they've rendered their mathematical formulas on the server. For example:
Page: https://brilliant.org/practice/distributive-property-misconceptions/
Source: view-source:https://brilliant.org/practice/distributive-property-misconceptions/
But as much as I know, when we use MathJax to render formulas, we should deliver them in original format, and it will render then client-side. And if we give MathJax the rendered formula, it throws many errors re-rendering them.
So, how brilliant.org has managed to render formulas on server-side?
Brilliant.org uses Katex library to render math formula. Katex offers server-side and client-side rendering. As I know, Brilliant renders on server side which is more efficient than rendering on the client-side.
There are several ready WYSWYG editors integrates Katex library. One of them is https://quilljs.com/. When you enter formula, it renders into the HTML editor.