We have already made fork for capybara-webkit and now we can test localStorage features using cucumber/capybara-webkit.
But for enabling properly this feature you should use the next following snippet in the env.rb:
Capybara.default_host = "lvh.me"
Capybara.app_host = "http://lvh.me:3003"
Capybara.server_port = 3003
Before('@html5') do
if [:webkit, :webkit_debug].include? Capybara.current_driver
page.driver.browser.set_attribute("LocalStorageEnabled")
# Single localStorage per scenario by generating subdomain
part = (1000 * rand()).to_i.to_s
Capybara.default_host = "subdomain#{part}.lvh.me"
Capybara.app_host = "http://subdomain#{part}.lvh.me:3003"
end
end