Not really defined, but since an object is defined as an unordered collection of key/value pairs, a conforming parser could probably shuffle the pairs before parsing them.
if not key in hash:
hash[key] = value
> I'm sure they didn't consider a parser randomly permuting the lexical order of the pairs as something a sane person would do.
It could sort the keys, in which case the order is no longer guaranteed (again this doesn't seem insane).
The proposal is to rely on undefined behaviour for comments. I'm amazed we're still talking about this.