Recently I searched for a way to use Emacs Lisp to write HTML, I found a library called “charge” from this guy which lets you do exactly that: https://hungyi.net/posts/dead-simple-emacs-lisp-templating/
The best way to get f=f (AKA format=flowed) working inside Mu4e on Emacs, is to put this code into a file, and then into load-path and require it.