There has been some work on a new Javascript implementation, as described in this thread: https://groups.google.com/d/msg/capnproto/lESKRE_pix8/jX59zE...

But it's been slow.

On the other hand, now that 0.6 includes a JSON library, it's relatively easy to do browser<->server in JSON and then use Cap'n Proto on the back-end. But, obviously, it would be nicer to use Cap'n Proto through the whole stack.

Contributions are welcome!