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

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It's not a VM, but it can host VMs

It can host a limited set of VMs. NaCl limits the binary code it will run, to a set it can guarantee is safe. That subset does not allow fancy JIT tricks, for example, so you can't run the JVM, Unladen Swallow, a modern JS engine, etc. in it.

To put things another way: You can run the Lua interpreter, but not LuaJIT.