A VM that provides a standardized bytecode and execution model capable of executing most modern dynamic and static turing complete languages with sufficient performance. This requires more general facilities for common functionality required; refer to tail call support, JVM's invokedynamic, etc.
Or, shorthand, a general purpose browser VM.
Unless, of course, you're arguing that the above is not truly possible, in which case I'd argue that's exactly what Google is exploring via PNaCL, and has (if not completely efficiently) been implemented across existing VMs. The PNaCL choice of LLVM bitcode was likely in error, but regardless, "no such thing" is a stretch.
Which comes full circle to my original point, which is that other than Google, nobody with a vested interest is trying, and worse yet, they actively oppose progress on this front.
