Hello,

This is a follow-up to the mails I sent to emacs-orgmode[1] and
help-gnu-emacs[2].

tl;dr I believe font-lock-{append,prepend}-text-property do not handle
anonymous faces correctly: they splice their attributes, producing
lists mixing keyword attributes and named faces such as this one:

    (:strike-through t org-level-1)

The patch I attach changes this to:

    ((:strike-through t) org-level-1)

To summarize my findings:

1. With org.el from Emacs's master branch (commit 88c91f53df), in an
   Org buffer with a heading such as this one:

        * *foo* /bar/ _baz_ +quux+
  
   foo (resp. bar and baz) show both the heading face and the bold
   (resp. italic and underline) decoration, whereas quux only shows
   the strike-through decoration, without the heading face.

2. C-u C-x = on quux says:

    > face        (:strike-through t org-level-1)

   A hasty look at org.el showed that it used
   `font-lock-prepend-text-property`.  I sent a trivial patch to
   emacs-orgmode which uses `font-lock-append-text-property` instead,
   although I wasn't sure that it was sound.  With this patch, C-u
   C-x = says instead:
   
    > face        (org-level-1 :strike-through t)

   … and quux shows both the org-level-1 foreground and the
   strike-through decoration.

3. On help-gnu-emacs, Stefan confirmed that my patch relies on
   undefined behavior.

4. Meanwhile, Nicolas applied it to the org-mode repository (commit
   42abf5c69).

I have now come to the conclusion that a patch such as the one I
attached in this report might be more appropriate.