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.

Not unless you want to compile the JVM to webassembly and load a it on every pageview.