I believe this is the first public and live Instant Articles feed.
As you can see it's also RSS 2.0, something I'm quite proud of.
When I was developing my Instant Articles feed, I desperately needed a working example to follow. It would have helped to have something that worked to study and copy. Now that my feed has been working reliably with Facebook's OS, I wanted to share it.
Interop is what matters. This is a good place to start.
PS: Is it really open? Well, you can write an application that uses that feed. (Please do.) So yes, it's open.