Comment 63 for bug 190848

I suggest that the best fix is to modify the system fontconfig settings, so
that they become defaults rather than always overriding settings already
present in the pattern.

This would include changing 10-no-sub-pixel.conf from

  <match target="font">
    <edit name="rgba" mode="assign"><const>none</const></edit>
  </match>

to

  <match target="font">
     <test name="rgba" qual="all"><const>unknown</const></test>
    <edit name="rgba" mode="assign"><const>none</const></edit>
  </match>

and changing 10-hinting-medium.conf from

  <match target="font">
    <edit name="hintstyle" mode="assign"><const>hintmedium</const></edit>
  </match>

to something like

  <match target="font">
    <test name="hintstyle" qual="all"><int>-1</int></test>
    <edit name="hintstyle" mode="assign"><const>hintmedium</const></edit>
  </match>

There are other files that should also be changed similarly.

Further discussion at
https://bugzilla.mozilla.org/show_bug.cgi?id=458612#c21