The thing is this goes both ways. For example, JSON with duplicate keys is valid JSON, and the standard specifies two ways to handle them. To support all ways JSON is used, an implementation will have to provide for both ways, and decide which one to pick as the default. The stricter default on the parsing side is at the same time the less interoperable one. Same for numeric precision: seamlessly allowing higher precision on the generating side lowers interoperability towards the receiver (but lessens demands on the driving application), while accepting it on the parsing side increases interoperability (but also places higher demands on the consuming application). So what do you do? There is no easy and simple answer, unless you want to limit the scope of applicability of your implementation.