If I recall correctly, [Waldemar Horwat] began by specifying a very precise, simple “spec language” with clear semantics. Then he wrote an interpreter for that language in Common Lisp. Then he wrote the E4 spec in his spec language. The result was that he could compile the specification itself into a working ECMAScript interpreter. Eric Lippert on Why is there a large difference in readability between the C# and ECMAScript specifications?