Why doesn't 'font-weight: 100' work yet?

It used to be that the font APIs on many operating systems weren't rich 
enough to put more than two weights in one family. That's no longer the 
case, but most browsers still don't seem to recognize any weights other 
than 'normal' or 'bold'. Why is that?

E.g., the fontconfig library on my system finds all three weights of 
Gill Sans, but my browser doesn't:

    $ fc-match "Gill Sans:weight=thin:slant=roman"        # 100
    GillSansLight.ttf: "Gill Sans" "Light"
    $ fc-match "Gill Sans:weight=extralight:slant=roman"  # 200
    GillSansLight.ttf: "Gill Sans" "Light"
    $ fc-match "Gill Sans:weight=light:slant=roman"       # 300
    GillSansLight.ttf: "Gill Sans" "Light"
    $ fc-match "Gill Sans:weight=book:slant=roman"        # 400/500
    GillSans.ttf: "Gill Sans" "Regular"
    $ fc-match "Gill Sans:weight=normal:slant=roman"      # 500
    GillSans.ttf: "Gill Sans" "Regular"
    $ fc-match "Gill Sans:weight=demibold:slant=roman"    # 600
    GillSansBold.ttf: "Gill Sans" "Bold"
    $ fc-match "Gill Sans:weight=bold:slant=roman"        # 700
    GillSansBold.ttf: "Gill Sans" "Bold"
    $ fc-match "Gill Sans:weight=extrabold:slant=roman"   # 800
    GillSansBold.ttf: "Gill Sans" "Bold"
    $ fc-match "Gill Sans:weight=black:slant=roman"       # 900
    GillSansBold.ttf: "Gill Sans" "Bold"

(Fontconfig uses slant instead of font-style and thin...black instead of 
100...900. We could discuss whether "book" maps to 400 or to 500, but 
for the rest the mapping is straightforward.)



Bert
-- 
  Bert Bos                                ( W 3 C ) http://www.w3.org/
  http://www.w3.org/people/bos                               W3C/ERCIM
  bert@w3.org                             2004 Rt des Lucioles / BP 93
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