An open source implementation is not the same as an open standard and is still not enough. By not having a separate standard oter browsers would either have to

An open source implementation is not the same as an open standard and is still not enough. By not having a separate standard oter browsers would either have to support bug-for-bug compatibility with NaCl or be forced to use the reference implementation without any changes (so it might as well have been distributed as a obfuscated binary).