--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -8209,24 +8209,29 @@ nsLayoutUtils::Initialize()
                                "nglayout.debug.invalidation");
   Preferences::AddBoolVarCache(&sInterruptibleReflowEnabled,
                                "layout.interruptible-reflow.enabled");
   Preferences::AddBoolVarCache(&sSVGTransformBoxEnabled,
                                "svg.transform-box.enabled");
   Preferences::AddBoolVarCache(&sTextCombineUprightDigitsEnabled,
                                "layout.css.text-combine-upright-digits.enabled");
 #ifdef MOZ_STYLO
+#ifdef MOZ_OLD_STYLE
   if (PR_GetEnv("STYLO_FORCE_ENABLED")) {
     sStyloEnabled = true;
   } else if (PR_GetEnv("STYLO_FORCE_DISABLED")) {
     sStyloEnabled = false;
   } else {
     Preferences::AddBoolVarCache(&sStyloEnabled,
                                  "layout.css.servo.enabled");
   }
+#else
+  sStyloEnabled = true;
+#endif
+
   // We should only create the blocklist ONCE, and ignore any blocklist
   // reloads happen. Because otherwise we could have a top level page that
   // uses Stylo (if its load happens before the blocklist reload) and a
   // child iframe that uses Gecko (if its load happens after the blocklist
   // reload). If some page contains both backends, and they try to move
   // element across backend boundary, it could crash (see bug 1404020).
   sStyloBlocklistEnabled =
     Preferences::GetBool("layout.css.stylo-blocklist.enabled");
@@ -8368,25 +8373,29 @@ nsLayoutUtils::RemoveFromStyloBlocklist(
     sStyloBlocklist = nullptr;
   }
 }
 
 /* static */
 bool
 nsLayoutUtils::StyloChromeEnabled()
 {
+#ifdef MOZ_OLD_STYLE
   static bool sInitialized = false;
   static bool sEnabled = false;
   if (!sInitialized) {
     // We intentionally don't allow dynamic toggling of this pref
     // because it is rather risky to mix style backend in XUL.
     sEnabled = Preferences::GetBool("layout.css.servo.chrome.enabled");
     sInitialized = true;
   }
   return sEnabled;
+#else
+  return true;
+#endif
 }
 #endif
 
 /* static */
 void
 nsLayoutUtils::RegisterImageRequest(nsPresContext* aPresContext,
                                     imgIRequest* aRequest,
                                     bool* aRequestRegistered)

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -5788,17 +5788,21 @@ pref("layout.css.stylo-blocklist.blocked
 pref("layout.css.servo.enabled", true);
 #else
 pref("layout.css.servo.enabled", false);
 #endif
 // Whether Stylo is enabled for chrome document?
 // If Stylo is not enabled, this pref doesn't take any effect.
 // Note that this pref is only read once when requested. Changing it
 // at runtime may have no effect.
+#ifdef MOZ_OLD_STYLE
 pref("layout.css.servo.chrome.enabled", false);
+#else
+pref("layout.css.servo.chrome.enabled", true);
+#endif
 #endif
 
 // TODO: Bug 1324406: Treat 'data:' documents as unique, opaque origins
 // If true, data: URIs will be treated as unique opaque origins, hence will use
 // a NullPrincipal as the security context.
 // Otherwise it will inherit the origin from parent node, this is the legacy
 // behavior of Firefox.
 pref("security.data_uri.unique_opaque_origin", true);

--- a/toolkit/moz.configure
+++ b/toolkit/moz.configure
@@ -545,40 +545,51 @@ id_and_secret_keyfile('Bing API')
 simple_keyfile('Adjust SDK')
 
 id_and_secret_keyfile('Leanplum SDK')
 
 simple_keyfile('Pocket API')
 
 # Servo integration
 # ==============================================================
-option('--enable-stylo', nargs='?', choices=('build',),
-       help='Include Stylo in the build and/or enable it at runtime')
+option('--enable-stylo', nargs='?', choices=('build', 'only'),
+       help='Include Stylo in the build.  "build" means to disable Stylo at ' +
+            'runtime, and "only" means to exclude the old style system from ' +
+            'the build.')
 
 @depends('--enable-stylo', '--help')
 def stylo_config(value, _):
     build_stylo = None
     enable_stylo = None
+    old_style = None
 
-    # If nothing is specified, default to building and enabling Stylo where it
-    # is known to work.
+    # If nothing is specified, default to building and enabling Stylo,
+    # and building the old style system.
     if value.origin == 'default':
         build_stylo = True
         enable_stylo = True
+        old_style = True
     elif len(value) and value[0] == 'build':
         # Build but disable by request.
         build_stylo = True
+        old_style = True
+    elif len(value) and value[0] == 'only':
+        # Disable the old style system.
+        build_stylo = True
+        enable_stylo = True
     elif bool(value):
         # Build and enable.
         build_stylo = True
         enable_stylo = True
+        old_style = True
 
     return namespace(
         build = build_stylo,
         enable = enable_stylo,
+        old_style = old_style,
     )
 
 option('--disable-stylo-build-bindgen',
        help='Disable build-time bindgen for Stylo')
 
 @depends(stylo_config, '--enable-stylo-build-bindgen', '--enable-compile-environment')
 def building_stylo_bindgen(stylo_config, bindgen_enabled, compile_environment):
     if not compile_environment:
@@ -784,16 +795,18 @@ with only_when(building_stylo_bindgen):
     set_config('MOZ_LIBCLANG_PATH', bindgen_config_paths.libclang_path)
     set_config('MOZ_CLANG_PATH', bindgen_config_paths.clang_path)
     set_config('MOZ_STYLO_BINDGEN', depends_if('--enable-stylo-build-bindgen')(lambda _: True))
 
 set_config('MOZ_STYLO', stylo_config.build)
 set_define('MOZ_STYLO', stylo_config.build)
 set_config('MOZ_STYLO_ENABLE', stylo_config.enable)
 set_define('MOZ_STYLO_ENABLE', stylo_config.enable)
+set_config('MOZ_OLD_STYLE', stylo_config.old_style)
+set_define('MOZ_OLD_STYLE', stylo_config.old_style)
 
 option('--with-servo', env='SERVO_TARGET_DIR', nargs=1,
        help='Absolute path of the target directory where libgeckoservo can '
             'be found. This is generally servo_src_dir/target/release.')
 
 @depends_if('--with-servo')
 def servo_target_dir(value):
     return value[0]