The step we used in the past (Then "foo" should not be visibile) doesn't reliably work in Selenium features.
I overhauled the entire step so it uses Javascript to detect visibility in Selenium.
The step has support for jQuery and Prototype projects, so it should be a drop-in replacement for all your projects.
For Rack::Test the step no longer uses XPath so you should be able to understand it when you are not a cyborg :)
There were some other cards detailing alternative steps to detect visibility. I deleted all these other cards since the updated step should work in all situations. If you find a problem with the updated step, please work with me to maintain it. We should only have one standard step for the same thing.

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Updated: Capybara: Check that a page element is hidden via CSS