Brendan has previously talked about some obstacles to a bytecode/VM-level specification, and some advantages of specifying the browser language at the source level rather than at the bytecode level:

http://news.ycombinator.com/item?id=1893686 (see comments by BrendanEich and others)

http://www.aminutewithbrendan.com/pages/20101122

http://www.hanselman.com/blog/JavaScriptIsAssemblyLanguageFo...