What would be the best headless web testing/automation library to use,
given that:
1. I want to automate filling in and submitting forms on one web site,
which uses only a little JavaScript.
2. I want something that balances ease of use and stability, and is
reasonably actively maintained
?

I've been playing around with Watir and gotten it to do what I want,
but it involves a live browser; I would like something headless. When
I asked on IRC, I was told Capybara was good -- and it does look good,
but I'm not sure if it has too many moving parts for my purposes.