Capybara.disable_animations can also accept a CSS selector

If it receives a falsey (false or nil) it won't do anything.
If it receives true, it will disable animations on the '*' css selector
If it receives a string, it will use that string as the css selector

It will still always disable css for jQuery if it's around.

lib/capybara/server/animation_disabler.rb

@@ -3,8 +3,20 @@
 module Capybara
   class Server
     class AnimationDisabler
+      def self.selector_for(css_or_bool)
+        case css_or_bool
+        when String
+          css_or_bool
+        when true
+          '*'
+        else
+          raise CapybaraError, 'Capybara.disable_animation supports either a String (the css selector to disable) or a boolean'
+        end
+      end
+
       def initialize(app)
         @app = app
+        @disable_markup = DISABLE_MARKUP_TEMPLATE % AnimationDisabler.selector_for(Capybara.disable_animation)
       end
 
       def call(env)
@@ -20,18 +32,20 @@ module Capybara
 
     private
 
+      attr_reader :disable_markup
+
       def html_content?
         !!(@headers['Content-Type'] =~ /html/)
       end
 
       def insert_disable(html)
-        html.sub(%r{(</head>)}, DISABLE_MARKUP + '\\1')
+        html.sub(%r{(</head>)}, disable_markup + '\\1')
       end
 
-      DISABLE_MARKUP = <<~HTML
+      DISABLE_MARKUP_TEMPLATE = <<~HTML
         <script defer>(typeof jQuery !== 'undefined') && (jQuery.fx.off = true);</script>
         <style>
-          * {
+           %s {
              transition: none !important;
              animation-duration: 0s !important;
              animation-delay: 0s !important;

lib/capybara/session/config.rb

@@ -86,9 +86,9 @@ module Capybara
     end
 
     remove_method :disable_animation=
-    def disable_animation=(bool)
-      warn 'Capybara.disable_animation is a beta feature - it may change/disappear in a future point version' if bool
-      @disable_animation = bool
+    def disable_animation=(bool_or_allowlist)
+      warn 'Capybara.disable_animation is a beta feature - it may change/disappear in a future point version' if bool_or_allowlist
+      @disable_animation = bool_or_allowlist
     end
 
     remove_method :test_id=

spec/shared_selenium_session.rb

@@ -325,12 +325,14 @@ RSpec.shared_examples 'Capybara::Session' do |session, mode|
 
     context 'AnimationDisabler' do
       before(:context) do # rubocop:disable RSpec/BeforeAfterAll
+        # NOTE: Although Capybara.SpecHelper.reset! sets Capybara.disable_animation to false,
+        # it doesn't affect any of these tests because the settings are applied per-session
         Capybara.disable_animation = true
         @animation_session = Capybara::Session.new(session.mode, TestApp.new)
       end
 
       after(:context) do # rubocop:disable RSpec/BeforeAfterAll
-        Capybara.disable_animation = false
+        @animation_session = nil
       end
 
       it 'should disable CSS transitions' do
@@ -344,6 +346,56 @@ RSpec.shared_examples 'Capybara::Session' do |session, mode|
         @animation_session.click_link('animate me away')
         expect(@animation_session).to have_no_link('animate me away', wait: 0.5)
       end
+
+      context 'if we pass in css that matches css on screen' do
+        before(:context) do # rubocop:disable RSpec/BeforeAfterAll
+          # NOTE: Although Capybara.SpecHelper.reset! sets Capybara.disable_animation to false,
+          # it doesn't affect any of these tests because the settings are applied per-session
+          Capybara.disable_animation = '#with_animation a'
+          @animation_session_with_matching_css = Capybara::Session.new(session.mode, TestApp.new)
+        end
+
+        after(:context) do # rubocop:disable RSpec/BeforeAfterAll
+          @animation_session_with_matching_css = nil
+        end
+
+        it 'should disable CSS transitions' do
+          @animation_session_with_matching_css.visit('with_animation')
+          @animation_session_with_matching_css.click_link('transition me away')
+          expect(@animation_session_with_matching_css).to have_no_link('transition me away', wait: 0.5)
+        end
+
+        it 'should disable CSS animations' do
+          @animation_session_with_matching_css.visit('with_animation')
+          @animation_session_with_matching_css.click_link('animate me away')
+          expect(@animation_session_with_matching_css).to have_no_link('animate me away', wait: 0.5)
+        end
+      end
+
+      context 'if we pass in css that does not matches css on screen' do
+        before(:context) do # rubocop:disable RSpec/BeforeAfterAll
+          # NOTE: Although Capybara.SpecHelper.reset! sets Capybara.disable_animation to false,
+          # it doesn't affect any of these tests because the settings are applied per-session
+          Capybara.disable_animation = '.this-class-matches-nothing'
+          @animation_session_without_matching_css = Capybara::Session.new(session.mode, TestApp.new)
+        end
+
+        after(:context) do # rubocop:disable RSpec/BeforeAfterAll
+          @animation_session_without_matching_css = nil
+        end
+
+        it 'should disable CSS transitions' do
+          @animation_session_without_matching_css.visit('with_animation')
+          @animation_session_without_matching_css.click_link('transition me away')
+          expect(@animation_session_without_matching_css).to have_link('transition me away', wait: 0.5)
+        end
+
+        it 'should disable CSS animations' do
+          @animation_session_without_matching_css.visit('with_animation')
+          @animation_session_without_matching_css.click_link('animate me away')
+          expect(@animation_session_without_matching_css).to have_link('animate me away', wait: 0.5)
+        end
+      end
     end
   end