I have a new test in my Rails arsenal: assert_valid_markup.

def assert_valid_markup(markup=@response.body)
  require 'net/http'
  response = Net::HTTP.start('validator.w3.org') do |w3c|
  query = 'fragment=' + CGI.escape(markup) + '&output=xml'
    w3c.post2('/check', query)
  end
  assert_equal 'Valid', response['x-w3c-validator-status']
end

Validating (X)HTML In Rails.