diff --git a/actiontext/app/helpers/action_text/content_helper.rb b/actiontext/app/helpers/action_text/content_helper.rb
index e63bd781df..0980bf0f00 100644
--- a/actiontext/app/helpers/action_text/content_helper.rb
+++ b/actiontext/app/helpers/action_text/content_helper.rb
@@ -4,7 +4,7 @@
 
 module ActionText
   module ContentHelper
-    mattr_accessor(:sanitizer) { Rails::Html::Sanitizer.safe_list_sanitizer.new }
+    mattr_accessor(:sanitizer) { Rails::Html::Sanitizer.best_supported_vendor.safe_list_sanitizer.new }
     mattr_accessor(:allowed_tags) { sanitizer.class.allowed_tags + [ ActionText::Attachment.tag_name, "figure", "figcaption" ] }
     mattr_accessor(:allowed_attributes) { sanitizer.class.allowed_attributes + ActionText::Attachment::ATTRIBUTES }
     mattr_accessor(:scrubber)
diff --git a/actiontext/lib/action_text.rb b/actiontext/lib/action_text.rb
index d6038a939f..6d6b1126ad 100644
--- a/actiontext/lib/action_text.rb
+++ b/actiontext/lib/action_text.rb
@@ -42,4 +42,18 @@ module Attachments
     autoload :Minification
     autoload :TrixConversion
   end
+
+  class << self
+    def html_document_class
+      return @html_document_class if defined?(@html_document_class)
+      @html_document_class =
+        defined?(Nokogiri::HTML5) ? Nokogiri::HTML5::Document : Nokogiri::HTML4::Document
+    end
+
+    def html_document_fragment_class
+      return @html_document_fragment_class if defined?(@html_document_fragment_class)
+      @html_document_fragment_class =
+        defined?(Nokogiri::HTML5) ? Nokogiri::HTML5::DocumentFragment : Nokogiri::HTML4::DocumentFragment
+    end
+  end
 end
diff --git a/actiontext/lib/action_text/fragment.rb b/actiontext/lib/action_text/fragment.rb
index 129344f90f..21a6897b8f 100644
--- a/actiontext/lib/action_text/fragment.rb
+++ b/actiontext/lib/action_text/fragment.rb
@@ -7,7 +7,7 @@ def wrap(fragment_or_html)
         case fragment_or_html
         when self
           fragment_or_html
-        when Nokogiri::HTML::DocumentFragment
+        when Nokogiri::XML::DocumentFragment # base class for all fragments
           new(fragment_or_html)
         else
           from_html(fragment_or_html)
@@ -30,7 +30,7 @@ def find_all(selector)
     end
 
     def update
-      yield source = self.source.clone
+      yield source = self.source.dup
       self.class.new(source)
     end
 
diff --git a/actiontext/lib/action_text/html_conversion.rb b/actiontext/lib/action_text/html_conversion.rb
index 1e1062ea3f..bef7730640 100644
--- a/actiontext/lib/action_text/html_conversion.rb
+++ b/actiontext/lib/action_text/html_conversion.rb
@@ -18,7 +18,7 @@ def create_element(tag_name, attributes = {})
 
     private
       def document
-        Nokogiri::HTML::Document.new.tap { |doc| doc.encoding = "UTF-8" }
+        ActionText.html_document_class.new.tap { |doc| doc.encoding = "UTF-8" }
       end
   end
 end
diff --git a/actiontext/test/integration/controller_render_test.rb b/actiontext/test/integration/controller_render_test.rb
index c9d8817d8f..afedaae17c 100644
--- a/actiontext/test/integration/controller_render_test.rb
+++ b/actiontext/test/integration/controller_render_test.rb
@@ -20,7 +20,7 @@ class ActionText::ControllerRenderTest < ActionDispatch::IntegrationTest
 
     host! "loocalhoost"
     get message_path(message, format: :json)
-    content = Nokogiri::HTML::DocumentFragment.parse(response.parsed_body["content"])
+    content = ActionText.html_document_fragment_class.parse(response.parsed_body["content"])
     assert_select content, "img:match('src', ?)", %r"//loocalhoost/.+/racecar"
   end
 
diff --git a/actiontext/test/integration/job_render_test.rb b/actiontext/test/integration/job_render_test.rb
index 86955b3794..d0f4211226 100644
--- a/actiontext/test/integration/job_render_test.rb
+++ b/actiontext/test/integration/job_render_test.rb
@@ -18,7 +18,7 @@ class ActionText::JobRenderTest < ActiveJob::TestCase
         perform_enqueued_jobs
       end
 
-      rendered = Nokogiri::HTML::DocumentFragment.parse(File.read(file))
+      rendered = ActionText.html_document_fragment_class.parse(File.read(file))
       assert_select rendered, "img:match('src', ?)", %r"//foo.example.com:9001/.+/racecar"
     end
   end