bug#51704: 29.0.50; set-fontset-font do not work well with :weight 'normal of font-spec

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Tue, 09 Nov 2021 08:33:04 +0100
> Cc: 51704@debbugs.gnu.org
> 
> tumashu  <tumashu@163.com> writes:
> 
> > medium work.
> >
> > Does old emacs support 'medium?
> 
> Yes, in older Emacsen, `medium' is an alias for `normal'.  This was
> split up since the font spec allows two different font weights here (and
> `medium' is heavier than `normal'), and Emacs didn't have a way to
> choose the actual `normal' font.
> 
> The problem is that if you have a font that doesn't have the now-lighter
> `normal' font, Emacs won't find the right spec.  I'm not sure whether
> there's any way to make a fallback of some kind here work...  I'm
> amending the text in etc/NEWS and moving it to the "incompatible"
> section, though.

Can we rethink this incompatible change?  Isn't there a way of
achieving the same goal without breaking people's font customizations?
Why was there a need to make 'normal' and 'medium' be different?