[mohamedattahri]

The thing is it's not a standard. It's a set of protocols and good practices. That's what makes it really difficult to limit in space.

But it's easy to know you're not going in the right direction when a choice of yours contradicts or duplicates something clearly mentioned in the specifications of one of these protocols.

For example, the ".json" suffix to URLs is a convention is Rails. How can a language/framework-agnostic API borrow conventions from a framework?

It's not about usability. It's about consistency which will impact developers perception of simplicity. They'll just have less work to do before they can actually get started using it.