Earlier quoted context omitted.
> - Can be extended as needed with new type specifiers. That's often not a good thing, at least if that allowance is given to third parties, as it wildly opens up the format to effective incompatibilities (fragmenting the ecosystem) and is a never ending source of security issues.
This assumes that if you don't include such capability, that applications won't find a way to extend it on their own regardless of the spec, and that's empirically false. There's a reason the vast majority of file formats do have some capacity for extension. By providing this capability you avoid much worse hacks (the devil you know and all that...) and you can ensure certain invariants are upheld (such as applicatio…
Except it's true. For a trivial example, json which does not offer these capabilities doesn't suffer from incompatibilities or security issues anywhere close to how much yaml does. Because a json file which is not POD requires a lot of additional documentation and code.
> There's a reason the vast majority of file formats do have some capacity for extension.
Yes, naïvety (if not outright brain damage): for a long long time it's been assumed that flexibility and extensibility were innocuous, and while we've known that was not at all the case for at least a generation, these ideas remain attractive nuisances.
More modern formats have tended to stay away from these, and those few which have not (like, once again, yaml) are case studies in why they should have.