The Validator will:
- complain if an enum property has a value that is not in the enumeration
- not complain about missing properties unless a property is required by the standard. This is because JSON usually won’t include a property for a null value
- not complain about extensions with properties that are not defined in the standard unless these extensions try to re-use existing properties that would cause issues
This means that local implementations can extend the standard and can implement proposed new properties in advance of them being added in a future version of the standard.
I’d recommend posting here about proposed extensions so others get chance to comment before you’re committed to them.