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I propose that :eq should be compatible with JQuery and use an zero based index.
Sorry, I should have added this in for reference, http://stackoverflow.com/questions/4124523/what-library-does-capybara-use-as-css-selector. I have seen a few mentions online of people experiencing the same surprise.
See https://github.com/tenderlove/nokogiri/issues/628 for related bug report.
Because this sort of change can break lots of things in bad ways, we're planning on addressing JQuery compatibility in 2.0. You can read more about the roadmap here: https://github.com/tenderlove/nokogiri/blob/master/ROADMAP.md
I'll add this issue to the roadmap.
Thanks for the reply, makes sense to make this part of a bigger update as you're right, it could certainly break things.