Demystifying Font Appearance on Linux : The Proper Way

"/etc/fonts hold Xft settings, while
.fonts.conf is a per-user override of
/etc/fonts/*
DEs set their settings using X
settings daemon (there are several of
them, one in each DE, and also
several standalone for use with
lightweight WMs)
applications get settings either from
Xft directly (/etc/fonts), or from X
settings daemon if it is active.
some applications (e.g. Google
Chrome) only get settings from X
settings daemon. so the only way to
get proper fonts in Chrome running
under lightweight WM is to run some
sort of xsettingsd
gtk and qt apps treat some of Xft
settings differently, so it can be
tricky to create fonts.conf that
works in the same way in Qt and GTK.

and here's what i think, but didn't really check:

GDK/Pango uses Xft with its default
settings, either from fonts.conf or X
settings.
Qt adds extra tweaks to fonts.conf
settings, which breaks some of the
settings.
Cairo seems to respect xft settings,
but rendering looks slightly
differently compared to Pango."