I did some further testing to answer your previous email, and I now think 
that my previous patch is a half-measure, and that TRT would be to simply 
ignore the font weight specified in the default face while searching for 
other fonts.  Consider the following recipe:

emacs -Q
M-: (fancy-startup-screen) RET

and now evaluate the following lines in turn:

(set-face-attribute 'default nil :font "Source Code Pro" :weight 'ultra-heavy) ;; 1
(set-face-attribute 'default nil :font "Source Code Pro" :weight 'heavy) ;; 2
(set-face-attribute 'default nil :font "Source Code Pro" :weight 'ultra-bold) ;; 3
(set-face-attribute 'default nil :font "Source Code Pro" :weight 'bold) ;; 4
(set-face-attribute 'default nil :font "Source Code Pro" :weight 'semi-bold) ;; 5
(set-face-attribute 'default nil :font "Source Code Pro" :weight 'medium) ;; 6
(set-face-attribute 'default nil :font "Source Code Pro" :weight 'normal) ;; 7
(set-face-attribute 'default nil :font "Source Code Pro" :weight 'semi-light) ;; 8
(set-face-attribute 'default nil :font "Source Code Pro" :weight 'light) ;; 9
(set-face-attribute 'default nil :font "Source Code Pro" :weight 'ultra-light) ;; 10
(set-face-attribute 'default nil :font "Source Code Pro" :weight 'thin) ;; 11

(If you don't have the Source Code Pro font on your system, I'm sure you 
can find another font with more weight variants with which you will 
observe a similar effect.)

With current master, the variable-pitch face is realized as follows:

- with 1-3: -ADBO-Source Code Pro-black-normal-normal-*-29-*-*-*-m-0-iso10646-1, which is a monospace font

- with 4: -PfEd-DejaVu Sans-bold-normal-normal-*-29-*-*-*-*-0-iso10646-1, which is a variable pitch font

- with 5: -ADBO-Source Code Pro-semibold-normal-normal-*-29-*-*-*-m-0-iso10646-1, which is again a monospace font

- with 6: -urw-nimbus sans l-regular-r-normal--29-210-100-100-p-158-iso8859-1, which is a variable pitch font but without anti-aliasing

- with 7: -PfEd-DejaVu Sans-regular-normal-normal-*-29-*-*-*-*-0-iso10646-1, which is a variable pitch font

- with 8-9: -ADBO-Source Code Pro-light-normal-normal-*-29-*-*-*-m-0-iso10646-1, which is again a monospace font

- with 10-11: -PfEd-DejaVu Sans-ultralight-normal-normal-*-29-*-*-*-*-0-iso10646-1, which is a variable pitch font

That can't be right.  Only 4, 7, and 10-11 correspond to what is expected 
for that face, namely a variable pitch font.  Fixing the 'medium' case (as 
my patch does) only improves 7.  Fixing the other cases would amount to do 
something similar for each possible weight (e.g. "also try 'bold' after 
trying 'ultra-bold'"), which in fact amounts to ignoring the weight in 
spec.

When the weight is ignored in font_find_for_lface, the variable-pitch face 
is realized as follows:

- with 1-5: -PfEd-DejaVu Sans-bold-normal-normal-*-29-*-*-*-*-0-iso10646-1

- with 6-7: -PfEd-DejaVu Sans-regular-normal-normal-*-29-*-*-*-*-0-iso10646-1

- with 8-11: -PfEd-DejaVu Sans-ultralight-normal-normal-*-29-*-*-*-*-0-iso10646-1

Which is clearly TRT.

New patch attached.