I've toyed with this idea before, though I had considered Lua due to its specialization in embedded environments and relatively simple sandboxing.
Interesting choice, here, in going with Ruby. Ruby is not exactly intended for embedding, and I imagine that a veritable mountain of holes in the script sandbox would need to be plugged before anything like this could see mainstream adoption. On the other hand, people are likely to be a lot more excited about native Ruby support in a browser than they would be about Lua.
Overall, I wish this project much success. It will be a long road, but I hope Ruby is just the first language to be supported in this way...
Mruby has been compiled with Emscripten. There's a demo of it on the Emscripten wiki, where you can edit and run Ruby client side that way, though obviously with the caveat that bridging from Emscripten generated code to "normal" javascript is far from ideal.
Interesting choice, here, in going with Ruby. Ruby is not exactly intended for embedding, and I imagine that a veritable mountain of holes in the script sandbox would need to be plugged before anything like this could see mainstream adoption. On the other hand, people are likely to be a lot more excited about native Ruby support in a browser than they would be about Lua.
Overall, I wish this project much success. It will be a long road, but I hope Ruby is just the first language to be supported in this way...