The typical (offline) typesetting pipeline in technical fields starts with content and markup in a *.tex input file, and ends with a LaTeX executable outputting a pdf. There are a number of options for bringing the expressiveness of LaTeX mathematical notation to web pages. KaTeX, a relatively recent arrival, focuses on simple embedding and fast rendering. It doesn't supports all LaTeX functionality, but covers a good subset. With the associated "auto-render" extension, unmodified LaTeX can be inserted directly into HTML between specified delimiters.

Here is Vandermonde's identity, a relation among binomial coefficients:
%% \binom{m + n}{r} = \sum_{k=0}^r \binom{m}{k} \binom{n}{r - k} %%