> I would have been happy to see NaCl-minus-Pepper standardized[...]

If there was no standard API, then sites would have a different API they had to use for each browser. Then we're back to the `UserAgent` days.

Unfortunately, I think it must be an all or nothing ordeal :/

> [...]browsers could then choose what superset of APIs to expose to that sandbox.

Pepper can already do that. All Pepper APIs are retrieved by calling a function provided to the module at start with a single string argument (ie `"PPB_Audio;1.1"` or `"PPB_Audio;1.0"` or `"PPB_FileSystem;1.0"`). It's impossible to use PPAPI without first getting the specific interface structures from that function.