> Thanks! In which branch should I add it, master or Emacs-26?

How large is it? Also, is it supposed to be used outside of the ert
context?

The main source file is about 45 KB, with additional 15KB of test files.

It can be used outside the ERT context, as the markup language is generic. I can think of many applications for it, like use it as the storage format for a word-processor like mode, where the user can apply bold, italics etc.

> I guess it should go into lisp/emacs-lisp, as "ert.el" is located there.

Probably. Actually, why not make it part of ert, say, ert-faces.el?

As it can be used outside of ert, I would prefer the current name.

-- Anders