Eventually it should be, but trying to design a way to integrate multiple runtimes into a single webpage is going to take some work (e.g. imagine a page that imports code originally written in Python and Javascript and Ruby and Go, all with their own garbage collectors (and Go's custom concurrency model!)). The reason that C and C++ and Rust will work from the start is that they don't require invasive runtimes and so allow the browser vendors to punt on this problem until later.