--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -295,16 +295,17 @@ bool nsContentUtils::sIsUnprefixedFullsc
 bool nsContentUtils::sTrustedFullScreenOnly = true;
 bool nsContentUtils::sIsCutCopyAllowed = true;
 bool nsContentUtils::sIsUpgradableDisplayContentPrefEnabled = false;
 bool nsContentUtils::sIsFrameTimingPrefEnabled = false;
 bool nsContentUtils::sIsPerformanceTimingEnabled = false;
 bool nsContentUtils::sIsResourceTimingEnabled = false;
 bool nsContentUtils::sIsPerformanceNavigationTimingEnabled = false;
 bool nsContentUtils::sIsFormAutofillAutocompleteEnabled = false;
+bool nsContentUtils::sIsUAWidgetEnabled = false;
 bool nsContentUtils::sIsShadowDOMEnabled = false;
 bool nsContentUtils::sIsCustomElementsEnabled = false;
 bool nsContentUtils::sSendPerformanceTimingNotifications = false;
 bool nsContentUtils::sUseActivityCursor = false;
 bool nsContentUtils::sAnimationsAPICoreEnabled = false;
 bool nsContentUtils::sGetBoxQuadsEnabled = false;
 bool nsContentUtils::sSkipCursorMoveForSameValueSet = false;
 bool nsContentUtils::sRequestIdleCallbackEnabled = false;
@@ -661,16 +662,19 @@ nsContentUtils::Init()
                                "security.mixed_content.upgrade_display_content", false);
 
   Preferences::AddBoolVarCache(&sIsFrameTimingPrefEnabled,
                                "dom.enable_frame_timing", false);
 
   Preferences::AddBoolVarCache(&sIsFormAutofillAutocompleteEnabled,
                                "dom.forms.autocomplete.formautofill", false);
 
+  Preferences::AddBoolVarCache(&sIsUAWidgetEnabled,
+                               "dom.ua_widget.enabled", false);
+
   Preferences::AddBoolVarCache(&sIsShadowDOMEnabled,
                                "dom.webcomponents.shadowdom.enabled", false);
 
   Preferences::AddBoolVarCache(&sIsCustomElementsEnabled,
                                "dom.webcomponents.customelements.enabled", false);
 
   Preferences::AddIntVarCache(&sPrivacyMaxInnerWidth,
                               "privacy.window.maxInnerWidth",

--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -3103,16 +3103,28 @@ public:
                                         nsContentPolicyType& aContentPolicyType,
                                         uint64_t* aRequestContextID);
 
   static nsresult
   CreateJSValueFromSequenceOfObject(JSContext* aCx,
                                     const mozilla::dom::Sequence<JSObject*>& aTransfer,
                                     JS::MutableHandle<JS::Value> aValue);
 
+  /**
+   * Returns whether or not UA Widget is enabled, controlled by pref
+   * dom.ua_widget.enabled.
+   *
+   * When enabled, UA Widget will replace legacy XBL when rendering JS-implemented
+   * web content widgets (videocontrols/datetimebox/etc.)
+   *
+   * It is really enabled only if Shadow DOM is also enabled.
+   */
+  static bool
+  IsUAWidgetEnabled() { return sIsShadowDOMEnabled && sIsUAWidgetEnabled; }
+
   static bool
   IsShadowDOMEnabled() { return sIsShadowDOMEnabled; }
 
   /**
    * Returns true if reserved key events should be prevented from being sent
    * to their target. Instead, the key event should be handled by chrome only.
    */
   static bool ShouldBlockReservedKeys(mozilla::WidgetKeyboardEvent* aKeyEvent);
@@ -3422,16 +3434,17 @@ private:
   static bool sIsCutCopyAllowed;
   static uint32_t sHandlingInputTimeout;
   static bool sIsPerformanceTimingEnabled;
   static bool sIsResourceTimingEnabled;
   static bool sIsPerformanceNavigationTimingEnabled;
   static bool sIsUpgradableDisplayContentPrefEnabled;
   static bool sIsFrameTimingPrefEnabled;
   static bool sIsFormAutofillAutocompleteEnabled;
+  static bool sIsUAWidgetEnabled;
   static bool sIsShadowDOMEnabled;
   static bool sIsCustomElementsEnabled;
   static bool sSendPerformanceTimingNotifications;
   static bool sUseActivityCursor;
   static bool sAnimationsAPICoreEnabled;
   static bool sGetBoxQuadsEnabled;
   static bool sSkipCursorMoveForSameValueSet;
   static bool sRequestIdleCallbackEnabled;

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -1368,16 +1368,22 @@ pref("privacy.trackingprotection.lower_n
 pref("privacy.trackingprotection.lower_network_priority", false);
 #endif
 
 pref("dom.event.contextmenu.enabled",       true);
 pref("dom.event.clipboardevents.enabled",   true);
 pref("dom.event.highrestimestamp.enabled",  true);
 pref("dom.event.coalesce_mouse_move",       true);
 
+#if defined(NIGHTLY_BUILD) && !defined(ANDROID)
+pref("dom.ua_widget.enabled", true);
+#else
+pref("dom.ua_widget.enabled", false);
+#endif
+
 #ifdef NIGHTLY_BUILD
 pref("dom.webcomponents.shadowdom.enabled", true);
 pref("dom.webcomponents.customelements.enabled", true);
 #else
 pref("dom.webcomponents.shadowdom.enabled", false);
 pref("dom.webcomponents.customelements.enabled", false);
 #endif