MathJax puts equations in webpages.

For example, a file like this:

<script src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js"></script>
$$ E = m c^2 $$

would be displayed like this:

\[E = m c^2\]

Right-click on the equation, and you will see lots of MathJax options, including Show Math As > TeX Commands, which shows the original E = m c^2.

See this excellent tutorial and quick reference.

In case you’re wondering, MathJax works by adding new DOM elements to the webpage after it is loaded.