Use Nokogiri::HTML5 to parse strings if nokogumbo is included

.travis.yml

@@ -89,6 +89,9 @@ matrix:
     - gemfile: Gemfile
       rvm: 2.5.1
       env: W3C=true HEADLESS=true
+    - gemfile: gemfiles/Gemfile.gumbo
+      rvm: 2.5.3
+      script: bundle exec rake spec_rack
   allow_failures:
     - gemfile: gemfiles/Gemfile.beta-versions
     - gemfile: gemfiles/Gemfile.edge-marionette

gemfiles/Gemfile.gumbo

@@ -0,0 +1,7 @@
+source 'https://rubygems.org'
+
+gem 'bundler', '~> 1.1'
+gemspec path: '..'
+
+gem 'xpath', :git => 'git://github.com/teamcapybara/xpath.git'
+gem 'nokogumbo'

lib/capybara.rb

@@ -366,9 +366,18 @@ module Capybara
     # @return [Nokogiri::HTML::Document]      HTML document
     #
     def HTML(html) # rubocop:disable Naming/MethodName
-      Nokogiri::HTML(html).tap do |document|
-        document.xpath('//textarea').each do |textarea|
-          textarea['_capybara_raw_value'] = textarea.content.sub(/\A\n/, '')
+      if Nokogiri.respond_to?(:HTML5) # Nokogumbo installed
+        Nokogiri::HTML5(html).tap do |document|
+          document.xpath('//textarea').each do |textarea|
+            # The Nokogumbo HTML5 parser already returns spec compliant contents
+            textarea['_capybara_raw_value'] = textarea.content
+          end
+        end
+      else
+        Nokogiri::HTML(html).tap do |document|
+          document.xpath('//textarea').each do |textarea|
+            textarea['_capybara_raw_value'] = textarea.content.sub(/\A\n/, '')
+          end
         end
       end
     end

lib/capybara/spec/spec_helper.rb

@@ -117,7 +117,8 @@ module Capybara
 
     def extract_results(session)
       expect(session).to have_xpath("//pre[@id='results']")
-      YAML.load Nokogiri::HTML(session.body).xpath("//pre[@id='results']").first.inner_html.lstrip
+      # YAML.load Nokogiri::HTML(session.body).xpath("//pre[@id='results']").first.inner_html.lstrip
+      YAML.load Capybara::HTML(session.body).xpath("//pre[@id='results']").first.inner_html.lstrip
     end
   end
 end

spec/rack_test_spec.rb

@@ -1,6 +1,12 @@
 # frozen_string_literal: true
 
 require 'spec_helper'
+nokogumbo_required = begin
+                       require 'nokogumbo'
+                       true
+                     rescue LoadError
+                       false
+                     end
 
 module TestSessions
   RackTest = Capybara::Session.new(:rack_test, TestApp)
@@ -231,6 +237,15 @@ RSpec.describe Capybara::RackTest::Driver do
   end
 end
 
+RSpec.describe 'Capybara::String' do
+  it 'should use gumbo' do
+    skip 'Only  valid if gumbo is included' unless nokogumbo_required
+    allow(Nokogiri).to receive(:HTML5).and_call_original
+    Capybara.string('<div id=test_div></div>')
+    expect(Nokogiri).to have_received(:HTML5)
+  end
+end
+
 module CSSHandlerIncludeTester
   def dont_extend_css_handler
     raise 'should never be called'