Adding to this, PNaCl relies heavily on Chrome's sandbox, so any third-party implementation of PNaCl would involve re-implementing large swaths of Chrome. It's

Adding to this, PNaCl relies heavily on Chrome's sandbox, so any third-party implementation of PNaCl would involve re-implementing large swaths of Chrome. It's not as portable as you'd think. WASM was designed from the ground up to not depend on any browser's implementation.