To be able to type $\LaTeX$ equations in websites, you need to add a few lines of html to the heading of your page. I use the configuration given below.

First, tell your webpage where to find MathJax:

<script type="text/javascript" id="MathJax-script" async
src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js">
</script>


But, before that (which is important), we need to configure it. I like it to be able to use symbols and environments from amsmath, so I include the tag tags: ams. Additionally, one drawback of writing $\LaTeX$ on the web like this is that MathJax doesn’t understand that many packages; many convenient macros from packages like physics are not accessible. Still, we can partially fix it by defining our own macros — and I was surprised how few I really used regularly.

<script>
MathJax = {
tex: {
inlineMath: [['$', '$'], ['\$$', '\$$']],
displayMath: [ ['$$','$$'], ['$','$'] ],
tags: 'ams',
macros: {
d: '{\\mathrm{d}}',
pdv: ['{\\frac{\\partial #1}{\\partial #2}}', 2],
vbi: ["\\boldsymbol{#1}", 1],
vb: ["\\mathbf{#1}", 1],
vu: ["\\hat{\\mathbf{#1}}", 1],
vui: ["\\hat{\\boldsymbol{#1}}", 1],
e: "{\\mathrm{e}}",
i: "{\\mathrm{i}}",
dv: ["{\\frac{d #1}{d #2}}", 2],
},
},
svg: {
fontCache: 'global'
}
};
</script>


e.g. To get a numbered equation, I write:

$$$$\left( \i \gamma^\mu \partial_\mu - m \right) \psi = 0.$$$$


which renders as:

$$$\left( \i \gamma^\mu \partial_\mu - m \right) \psi = 0.$$$

Note that html wants the  around the $$...$$— it needs to know you’re trying to type $\LaTeX$.