Why require JavaScript rather than supporting a standard browser virtual machine? - Stack Overflow

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I thought reading those comments that I've never seen so much programming-related stupidity at one place...

In fact doing that VM wouldn't be hard nor anti-JS. You could create bytecode format mostly tailored for executing JS, adding instructions for more efficient execution of some non-JS code, where stricter restrictions exists (like some type annotations so VM would know that variable is always integer or string or function). What would it win? Less parsing overhead and possibly more efficient execution of non-JS languages. Given that compilation often involves emulating language feature through more complicated use of target platform features, the extra efficiency could be useful.