I couldn't find any solutions that use native controls on all platforms, but I found a few that at least don't try to do their own text rendering on Web, so they should work with a screen reader:
Flutter (Dart language) - Desktop and Web support is currently experimental, but it works
If you are open to learning how to use Emacs or Vim, they offer exactly what you want thanks to ediff and vimdiff respectively:
a graphical interface with clear color highlighting of the alternative versions of conflicts (but without removing any of the Git conflict markers)
keybindings to easily jump from one conflict to the next
keybindings to easily ...
Take a look at [Boa Constrictor]http://boa-constructor.sourceforge.net/). Also, Google - I can't remember the name, but a few years back I found several free apps which would generate a GUI if your Python code uses argparse.
Aha, Google says Gooey (see also here).
Possibly https://github.com/codypiersall/cligui, although, on its GitHub page, the author ...