/etc/fonts.conf allows HACKS to font substitution
it can break applications or even cause lock-ups. it can fix broken applictions: aha ... but which are you breaking or helping wen you hack it?
Linux From Scratch GNU/Linux x-lfs-2010
do NOT bother downloading. but the above supports Mathematica 4.0. the trick was INSTALLING FONTS CORRECTLY. some hackers had put bogus font substutitions in the X11 font alias files. those removed: order was critical. after all was fixed the math was right: no missing symbols, no wrong symbols, and for math you preettty much want PERFECT.
xorg your not going to get perfect. it's a sloppy system of substitution and never having the right font unless:
#1 its a gnu app that relies on fonts "just being there" and they are when, but (when someone hacks that font or changes the default in the default distro: your f'ed)
#2 the app is an "xorg" app and expects sloppy font choice, calculates and understands "guess choices", and somehow always shows you the right font: HIGHLY UNLIKELY. perhaps supports unicode %100. HIGHLY UNLIKELY.
#3 the app was shipped with all fonts (ie, installed Adobe font pack)
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Here's the issue. check this "A" here. it's a spline curve. but there are many rules just for that simple glyph (download a GPL font maker, you'll see it's complicated). the spline curve has to be coded and there are many formats
in the end: an app MUST ask for "A" in a table and it can crash if that "A" does not fit exactly in a box (or it will look _bad__fonts_ if it don't crash)
guess what: if that get's substituted how does the app know the substitution isn't bogus?
IT DOESN'T. THAT'S THE PROBLEM.
you have hacks tampering with very sensitive spline files, mixing the order in whcih they are found, then also IMPROPERLY simply substituting a WRONGLY sized shape glyph which might actually be THE WRONG MATH if the OS doesn't have the font asked for (ie, it's free fonts but app asked for a font using Adobe names)
solution: there isn't any really.
in the above URL only "correct" X11 fonts were installed, the FONTPATH was set to be well ordered, and the app fonts configured to be correctly found, and the app never missed a glyph. THE WEB BROWSER was (is) set to only use Courier fonts (which actually made Firefox run a ton faster btw)
here's another for example: you install gnoome but not all gnome fonts. you run gedit(1). crashes when you change fontsize. why? due to the substitution being the wrong size, gedit allocates the wrong memory, it don't fit and when the lines all come in: memory error, gedit "crashes", due to the bad substitution. why did gedit do that? #1 gedit is unsophisticated. #2 on the developer machine they have fonts installed by ubuntu (w/ubuntu font config, which ... ugh). they don't care if you don't. #3 the developers never have the problem and don't care if on other systems it breaks: your problem