Eli Zaretskii <eliz@gnu.org> writes:

 >> Date: Fri, 30 Jun 2023 11:05:33 -0300
 >> From: Mauro Aranda <maurooaranda@gmail.com>
 >> Cc: Stephen Berman <stephen.berman@gmx.net>, Eli Zaretskii 
<eliz@gnu.org>
 >>
 >> To sum it up, I think there are bugs in custom-face-attributes. One is
 >> most surely a typo, and the other ones are oversights in the filters for
 >> the :underline and :box properties.  Fixing those, we are left with one
 >> bug, I think, that will be reproducible with Emacs -Q and evaluating:
 >>
 >> (defface test
 >>    '((((supports :underline (:style wave)))
 >>       :underline (:color "Red1" :style wave))
 >>      (t
 >>       :inherit warning))
 >>    "...")
 >>
 >> (defface test-2
 >>    '((((supports :underline (:style wave)))
 >>       :underline (:style wave :color "Red1"))
 >>      (t
 >>       :inherit warning))
 >>    "...")
 >>
 >> M-x customize-face RET test
 >> will show STANDARD state
 >>
 >> while
 >> M-x customize-face RET test-2
 >> will show EDITED state
 >
 > Thanks.
 >
 > Can you show a patch for the two bugs you've succeeded to identify?
 >
 > Did you make any progress with the one bug that's left after the other
 > two are fixed?

Here's a patch for the typo.

Concerning the other bugs I discovered, I think that while the filters
could be tweaked, a better fix would be to teach face-spec-match-p
about matching plists correctly and not just by equality.