Support for color fonts [[https://lists.gnu.org/archive/html/emacs-devel/2019-04/msg00996.html][has landed]] on version 27.0 (master).
Remaining issues:

* DONE Fix font-hinting =--with-cairo=

Bug#35781 fixed by Yamamoto Mitsuharu.

* TODO Check whether combining sequences work

For example: regional flags, ZWJ sequences from

- ZWJ sequences from
  <https://unicode.org/emoji/charts/emoji-zwj-sequences.html>:
  - 🧟‍♂️, 🧟‍♀️
  - 🐕‍🦺
- regional indicators: 🇪🇺, 🇫🇷

* DONE Add a convenient way to set a font for all emojis

Robert Pluim added an =emoji= script that can be used with
~set-fontset-font~ in Emacs 28.