For Pepper? There is not, in fact. There's some developer documentation, but nothing resembling an actual implementation spec.

The code is open source, but last I heard there isn't a spec for PNaCl/PPAPI.

The PNaCl ABI is defined here: https://developers.google.com/native-client/dev/reference/pn... - and it's definitely a work in progress we'll be trying to improve. As for Pepper, it's also all documented here and below - https://developers.google.com/native-client/dev/pepperc/

Granted, this is documentation, not a standards-body spec.

Yes, certainly has had effort put into proper documentation. Do you know if there is also work being done to spec it formally?

We're definitely considering it.

Is there a PNaCl spec or just Google's implementation?

There's a bitcode spec [1] and a stability doc [2].

1 - https://developers.google.com/native-client/dev/reference/pn...

2 - http://www.chromium.org/nativeclient/pnacl/stability-of-the-...