The ECMAScript spec is actually quite thorough and freely available to read online. In addition, it has a very large suite of tests to check for compliance.
I don't think there's any other language with as many independent production-quality implementations.
In what way do you think it could be better specified?
Sure, but the number might fool you. Not every C++ compiler is standards-compliant, and at least on the PC side, many of the good compilers bought the frontend from the Edison Design Group and focussed on the backend.
Anywhere the spec says "implementation-dependent" or "behavior is undefined" could be fully specified. Some might say its impractical, but given the effort folks but into the hardware and software of a NASA project, you can really see the difference.
I don't think there's any other language with as many independent production-quality implementations.
In what way do you think it could be better specified?