gistfile1.txt

in Gemfile:
gem 'w3c_validators'

in spec/support/w3c_validator.rb:
include W3CValidators 

def validate_url(url)
  visit url
  result = MarkupValidator.new.validate_text(page.body)
  result.errors.should == []
end

in spec/acceptance/w3c_valid_spec.rb:
require File.dirname(__FILE__) + '/acceptance_helper'

feature "W3C Validation" do

  before :all do
    Capybara.use_default_driver
    Capybara.run_server = true
  end

  scenario "HTML" do
    validate_url '/'
  end

  # scenario "feed XML" do
  # end

end