I've written JSON parsers to replace platform specific JSON parsers with bug-for-bug (or at the very least misfeature-for-misfeature) parity to port code without breaking it, without too much going terribly wrong. I wouldn't even try to attempt the same for XML.

Generating a useful conservative subset of JSON that most/all JSON serializers will accept hasn't been that hard in practice IME (no trailing commas, escape all unicode, don't assume >double precision/range scalars, etc.), but I still haven't figured out how to do the same for some XML serializers (failing to serialize because it lacks 'extra' annotation tags in some cases, failing to serialize because it doesn't ignore 'extra' annotation tags in other cases...)