Comment by projektfu

FWIW, org's HTML export is about 4000 lines of elisp, and it is pretty well modularized.

https://git.sr.ht/~bzg/org-mode/tree/main/item/lisp/ox-html....

The publish front end is in

https://git.sr.ht/~bzg/org-mode/tree/main/item/lisp/ox.el

and

https://git.sr.ht/~bzg/org-mode/tree/main/item/lisp/ox-publi...