Use MathJax like Upmath is doing it

[BaroDevelopment]

Would be cool if we could use MathJax as in this website https://upmath.me/

things like this are not working:

$$T^{\mu\nu}=\begin{pmatrix} \varepsilon&0&0&0\\ 0&\varepsilon/3&0&0\\ 0&0&\varepsilon/3&0\\ 0&0&0&\varepsilon/3 \end{pmatrix},$$