I would directly write this as a list stixsizedaltfonts = ["STIXGeneral", ...].
It's more readable than splitting a string. Also, you can wrap within the list and don't need the ugly line continuation char 😄. (追記ここまで) See dict suggestion below.

Since the numbering is essential, have you thought about turning stixsizedaltfonts into a dict, similar to what is done in DejaVuFonts?

I went for splitting a string here because that is also done in line 770.

for texfont in "cal rm tt it bf sf".split():
# ...

But of course, there is no line break and the order does not matter. So, I agree that a dict is more appropriate here. I also made sure to add the STIX alternatives with their names to the fontmap

self.fontmap[name] = fullpath

Not sure whether this is really necessary, but it is also the case for the DejaVuFonts.

Valid values of s must be a string. This check can be left out and you can just if s.lower() in fallback_fonts:. If you want to provide a better error message in case the user passes some other object, it's better to explicitly

if not isinstance(s, str):
 raise ValueError("'mathtext.fallback' must be a string.")

I changed this. Not sure, I understand this completely, since validate_fontsize has a similiar check/conversion. But maybe I am missing a subtle point here.

def validate_fontsize(s):
 fontsizes = ['xx-small', 'x-small', 'small', 'medium', 'large',
 'x-large', 'xx-large', 'smaller', 'larger']
 if isinstance(s, str):
 s = s.lower()
 if s in fontsizes:
 return s
 try:
 return float(s)
 except ValueError:
 raise ValueError("%s is not a valid font size. Valid font sizes "
 "are %s." % (s, ", ".join(fontsizes)))

validate_fontsizes does also accept numbers.

Ok, I see. But I might want to go with your initial suggestion then. Currently, the following code

mpl.rcParams['mathtext.fallback'] = 1

leads to the error

AttributeError: 'int' object has no attribute 'lower'

Of course the user should only supply a string, but the error message might not be very helpful in finding a potential bug in your own code accessing mpl.rcParams['mathtext.fallback'].

I added the following

if not isinstance(s, str):
 raise ValueError("Must be a string or None.")

which yields the following error message

ValueError: Key mathtext.fallback: Must be a string or None.

I hope that's ok.

This needs some documentation. In particular it seems fallback overrides fallback_to_cm if its set; there should be a comment in here.

Preferably, we would deprecate fallback_to_cm and make cm the default for mathtext.fallback.

This needs to be in matplotlibrc.template as well, right?