Yeah. And I learned this the hard way with the Perl module JSON::XS. It successfully encodes a Perl NaN, but its decoder will choke on that JSON. (Reported it to the maintainer who insists that is consistent with the documentation and wouldn't fix it)
My parser/serializer does the same.
How could that be fixed? Anything better to serialize NaN to than NaN? Or parse NaN?
So if a parser sees "Inifinity", which it doesn't have any concept of, why would it do anything except treating that as string of a word "Infinity"?
I just added those great testcases, and found previously unknown bugs. But the python test runner from this repo gives a few false negatives, PR coming soon. It was much easier to write it in perl.