A custom virtual machine probably isn't necessary. There are plenty of flash decompilers which can emit AS3 source from bytecode — and since ActionScript 3.0 is

A custom virtual machine probably isn't necessary. There are plenty of flash decompilers which can emit AS3 source from bytecode — and since ActionScript 3.0 is basically the old ECMAScript 6 draft spec, it can be trivially downcompiled to plain old Javascript using Mascara (http://ecmascript4.com/ http://ecmascript4.com/). Ideally the conversion would take place on a server-side proxy, and its ouput would subsequently be cached, but it could theoretically be done on the client as well. Re-implementing the standard library remains a major issue, but certainly not insurmountable.