Add a header that tells Internet Explorer (all versions) to use the best available standards support. This ensures that IE doesn't go into quirks mode because it has been blacklisted by too many users pressing the incompatible button. It also tells IE to use the ChromeFrame renderer, if the user has installed the plugin.

This guarantees that the best available standards support will be used on the client.

actionpack/lib/action_dispatch.rb

@@ -47,6 +47,7 @@ module ActionDispatch
   end
 
   autoload_under 'middleware' do
+    autoload :BestStandardsSupport
     autoload :Callbacks
     autoload :Cookies
     autoload :Flash

actionpack/lib/action_dispatch/railtie.rb

@@ -7,10 +7,11 @@ module ActionDispatch
     config.action_dispatch.x_sendfile_header = ""
     config.action_dispatch.ip_spoofing_check = true
     config.action_dispatch.show_exceptions = true
+    config.action_dispatch.best_standards_support = true
 
     # Prepare dispatcher callbacks and run 'prepare' callbacks
     initializer "action_dispatch.prepare_dispatcher" do |app|
       ActionDispatch::Callbacks.to_prepare { app.routes_reloader.execute_if_updated }
     end
   end
-end
+end

railties/lib/rails/application.rb

@@ -205,6 +205,7 @@ module Rails
         middleware.use ::ActionDispatch::ParamsParser
         middleware.use ::Rack::MethodOverride
         middleware.use ::ActionDispatch::Head
+        middleware.use ::ActionDispatch::BestStandardsSupport if config.action_dispatch.best_standards_support
       end
     end
 

railties/lib/rails/commands.rb

@@ -70,4 +70,4 @@ In addition to those, there are:
 
 All commands can be run with -h for more information.
   EOT
-end
+end

railties/lib/rails/engine/configuration.rb

@@ -50,4 +50,4 @@ module Rails
       end
     end
   end
-end
+end

railties/test/application/routing_test.rb

@@ -40,6 +40,7 @@ module ApplicationTests
 
       get '/foo'
       assert_equal 'foo', last_response.body
+      assert_equal "IE=Edge,chrome=1", last_response.headers["X-UA-Compatible"]
     end
 
     test "simple controller with helper" do