[riperoni]

The cost is not that high. There are plenty static site generators or things like pandoc. For example: https://pianomanfrazier.com/post/write-a-book-with-markdown/

You are asking for a responsive weblayout, which excludes any format that LaTeX outputs. Latex is a typesetting engine that does the job before presenting anything and not while presenting it. Its layouts are not responsive by design.

So your question if there is something like LaTeX for frontend is just misplaced, because the working method of LaTex does not fit into a responsive weblayout frontend. What LaTeX features do you want in a frontend? How do you want to write code and how do you want the layout to happen?

What you describe sounds more like static site generation with math handling and perhaps references. There are markdown extensions that can handle such things. For rendering there is e.g. https://www.mathjax.org/