We enjoy reading Miller's background on Language Virtuallizability and Lessons from TC39 History page 
> Turing universality guarantees that any universal machine can perfectly emulate any other machine; so how can "virtualizability" be a crisp distinction? Again following Popek and Goldberg page , this is why without rewriting is the important litmus test. The x86 is not a fully virtualizable architecture. Nevertheless, QEMU and VMWare can perfectly emulate an x86 on an x86, but only by complicated, expensive, and fragile means equivalent to interpretation or rewriting. Because EcmaScript is so expensive to parse and so hard to parse accurately, we have an even greater need to uphold full virtualizability.