Thanks for your effort, Ihor

But that code is so difficult for me, that I can barely understand it.

Ypo <ypuntot@gmail.com> writes:

I wanted to export my # comments so I could share my notes with more people, using HTML export. I would export all of them.

But, as it seems not possible, I think I will use footnotes. I liked # comments more, because their face can be customized and because they are nearer to the commented text (although footnotes can be moved manually wherever user wants to), # comments are like more "natural".

It is actually possible, but you will need to write an
org-export-before-parsing-hook that will convert all the comments into
exportable elements.

Not tested, but the hook might be something like

(defun org-export-replace-comments (_)
"Replace all the comments with note blocks."
  (org-element-cache-map
   (lambda (comment)
     (setf (buffer-substring (org-element-property :begin comment) (org-element-property :end comment))
	   (format "#+begin_note\n%s\n#+end_node\n" (org-element-property :value comment))))
   :granularity 'element
   :restrict-elements '(comment)))

Best,
Ihor