[0] https://github.com/dotnet/corert/blob/master/Documentation/h...
[1] http://www.mono-project.com/news/2017/08/09/hello-webassembl...
As I understand, Mono is going to be moving forwards with WebAssembly much faster than CoreRT will do. Mono has officially committed to it already, whereas for CoreRT it was more of an exploratory spike.
Also it makes much more sense for Mono to be the .NET runtime for WebAssembly. Mono is already Microsoft's preferred production-grade .NET runtime for most client scenarios (iOS/Android native apps, Unity games) and is designed for that kind of portability. WebAssembly is another client platform like those.