Katex produces MathML. The problem katex solves is that MathML is really ungainly for authoring equations. So instead you write equations in a DSL (which most people just call latex) and Katex compiles that to HTML/MathML for you.

You can do this server side or client side and sadly too many people do it client side. If you do it server side, it is just one more step in your build next to transpiling and bundling.

Katex can optionally produce MathML. By default it places individual symbols in a sea of <span> tags (due to inconsistent MathML output, I gather)