> if you do what the various specs on the internet suggest you do if you have an integer > 2^53: you encode it as decimal value as a string.

Hexadecimal is better - more compact, much less encoding/decoding effort. Alternatively, array of two integers does as well.

Text forms for any error, not NaN, are also useful.

But issues with non-UTF-8 encodings - as in CJK world - may spoil the picture pretty more.