A bit of a bodge way to test JS fallback behaviour on old browsers like IE8, using PhantomJS.

ie8_spoofer.js

navigator.geolocation        = undefined;
window.FormData              = undefined;
window.getComputedStyle      = undefined;
window.matchMedia            = undefined;
window.FileReader            = undefined;
window.Worker                = undefined;
window.WebSocket             = undefined;
window.requestAnimationFrame = undefined;
window.performance           = undefined;
window.EventSource           = undefined;
window.File                  = undefined;
window.FileError             = undefined;
window.FileList              = undefined;
window.FileReader            = undefined;
window.FileReaderSync        = undefined;
window.MSStream              = undefined;
window.MSStreamError         = undefined;
window.MSStreamReader        = undefined;
window.ObjectURLOptions      = undefined;
window.URL                   = undefined;

some_feature_spec.rb

context 'using an old browser (IE8)', driver: :poltergeist_pretend_ie8 do
  pending 'still works'
end

spec_helper.rb

# Poltergeist with some JS APIs removed to pretend to sniffers that it's IE8
Capybara.register_driver :poltergeist_pretend_ie8 do |app|
  Capybara::Poltergeist::Driver.new(app,
    extensions: ['spec/support/ie8_spoofer.js']
  )
end