The SimpleMathJax extension enables MathJax, a Javascript library, for typesetting TeX formula in MediaWiki inside math environments.

https://www.mediawiki.org/wiki/Extension:SimpleMathJax

Setting name	Default value	Description
$wgSimpleMathJaxSize	100	The default font size for SimpleMathJax
$wgSimpleMathJaxChem	false	enable <chem> tag
$wgSimpleMathJaxJs	"//cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?config=TeX-AMS-MML_HTMLorMML"	URL path of Math.js
$wgSimpleMathJaxChemJs	"//cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/extensions/TeX/mhchem.js"	URL path of mhchem.js

require "$IP/extensions/SimpleMathJax/SimpleMathJax.php";

To change font size, set $wgSimpleMathJaxSize.

require "$IP/extensions/SimpleMathJax/SimpleMathJax.php";
$wgSimpleMathJaxSize = 110;

To use chem tag, set $wgSimpleMathJaxChem.

require "$IP/extensions/SimpleMathJax/SimpleMathJax.php";
$wgSimpleMathJaxSize = 120;
$wgSimpleMathJaxChem = true;

To use local scripts, git submodule update and set variables.

$ cd /path/to/extensions/SimpleMathJax
$ git submodule init
$ git submodule update

require "$IP/extensions/SimpleMathJax/SimpleMathJax.php";
$wgSimpleMathJaxSize = 115;
$wgSimpleMathJaxChem = true;
$wgSimpleMathJaxJs = "{$wgScriptPath}/extensions/SimpleMathJax/MathJax/MathJax.js?config=TeX-AMS-MML_HTMLorMML";
$wgSimpleMathJaxChemJs = "{$wgScriptPath}/extensions/SimpleMathJax/MathJax-mhchem/mhchem.js";