test_stripe.rb

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'
  gem 'selenium-webdriver', '3.142.3'
  gem 'webdrivers'
  gem 'capybara', '3.28.0'
  gem 'byebug'
end

require "capybara/dsl"
require 'byebug'

sess = Capybara::Session.new(:selenium_chrome)
sess.visit("https://stripe.com/docs/stripe-js/elements/quickstart")
sess.within_frame do # only one outer frame - no locator necessary
  frame = sess.find('#card-element iframe')
  sess.within_frame(frame) do
    sess.fill_in('cardnumber', with: '4242' * 4)
    sess.fill_in('exp-date', with: '1220')
    sess.fill_in('cvc', with: '111')
    sess.fill_in('postal', with: '92054')

    sess.assert_selector(:field, 'cardnumber', with: '4242 4242 4242 4242')
    sess.assert_selector(:field, 'exp-date', with: '12 / 20')
    sess.assert_selector(:field, 'cvc', with: '111')
    sess.assert_selector(:field, 'postal', with: '92054')
  end
end

sleep 30 # delay to look at browser