This is really cool!
I'd love to see the "Hello World" example template provided for all languages, not just C (or at least, for Rust as well, since the C++ version would be awfully close to the C one already).
EDIT: On a different note, I was confused when I loaded a template, hit "Run", and got nothing. I assumed running would automatically build first. It would be nice to somehow hint to the user if they run something that hasn't been built yet.
In my understanding, part of why they don't have that in Rust yet is the lack of Cargo support; you'd want to use https://github.com/rustwasm/wasm-bindgen (and its README.md has said hello example in it) but there's no easy way to use it just yet. It's coming though.