On some systems (e.g. my current GNU/Linux system), "A" followed by
U+0308 COMBINING DIAERESIS isn't displayed as a single glyph.  See the
attached screenshot, which compares this character sequence with the
canonically-equivalent precomposed form "Ä".  The precomposed glyph
form looks better, and it would be great if Emacs could use it for the
composite variant as well.  Also see
https://lists.gnu.org/archive/html/help-gnu-emacs/2020-12/msg00898.html,
https://lists.gnu.org/archive/html/help-gnu-emacs/2021-01/msg00409.html,
https://lists.gnu.org/archive/html/help-gnu-emacs/2021-02/msg00808.html,
and
https://lists.gnu.org/archive/html/help-gnu-emacs/2021-03/msg00114.html
for some background discussion.