IL2CPP seeks to provide similar functionality as AOT platforms supported by mono today (e.g. iOS). If something works on iOS, it has a very good chance of running on an IL2CPP supported platform.

The exported user scripts are actually .Net assemblies. They are run via the mono VM using it's JIT (just in time) compiler on platforms that support it, and are AOT (ahead of time) compiled on platforms that don't like iOS.

In the case of WebGL the .Net assemblies are processed by a tool to convert them to C++ and then compiled into javascript using emscripten.