--- a/widget/gtk/WidgetStyleCache.cpp
+++ b/widget/gtk/WidgetStyleCache.cpp
@@ -139,18 +139,16 @@ CreateWidget(WidgetNodeType aWidgetType)
     case MOZ_GTK_MENUBAR:
       return CreateMenuBarWidget();
     case MOZ_GTK_MENUPOPUP:
       return CreateMenuPopupWidget();
     case MOZ_GTK_MENUBARITEM:
       return CreateMenuItemWidget(MOZ_GTK_MENUBAR);
     case MOZ_GTK_MENUITEM:
       return CreateMenuItemWidget(MOZ_GTK_MENUPOPUP);
-    case MOZ_GTK_TOOLTIP:
-      return CreateTooltipWidget();
     default:
       /* Not implemented */
       return nullptr;
   }
 }
 
 GtkWidget*
 GetWidget(WidgetNodeType aWidgetType)
@@ -161,18 +159,19 @@ GetWidget(WidgetNodeType aWidgetType)
     sWidgetStorage[aWidgetType] = widget;
   }
   return widget;
 }
 
 GtkStyleContext*
 CreateStyleForWidget(GtkWidget* aWidget, GtkStyleContext* aParentStyle)
 {
-  GtkWidgetPath* path =
-    gtk_widget_path_copy(gtk_style_context_get_path(aParentStyle));
+  GtkWidgetPath* path = aParentStyle ?
+    gtk_widget_path_copy(gtk_style_context_get_path(aParentStyle)) :
+    gtk_widget_path_new();
 
   // Work around https://bugzilla.gnome.org/show_bug.cgi?id=767312
   // which exists in GTK+ 3.20.
   gtk_widget_get_style_context(aWidget);
 
   gtk_widget_path_append_for_widget(path, aWidget);
   // Release any floating reference on aWidget.
   g_object_ref_sink(aWidget);
@@ -271,23 +270,19 @@ GetCssNodeStyleInternal(WidgetNodeType a
       gtk_style_context_add_class(style, GTK_STYLE_CLASS_BACKGROUND);
       break; 
     default:
       // TODO - create style from style path
       GtkWidget* widget = GetWidget(aNodeType);
       return gtk_widget_get_style_context(widget);
   }
 
-  if (style) {
-    sStyleStorage[aNodeType] = style;
-    return style;
-  }
-
-  MOZ_ASSERT_UNREACHABLE("missing style context for node type");
-  return nullptr;
+  MOZ_ASSERT(style, "missing style context for node type");
+  sStyleStorage[aNodeType] = style;
+  return style;
 }
 
 static GtkStyleContext*
 GetWidgetStyleWithClass(WidgetNodeType aWidgetType, const gchar* aStyleClass)
 {
   GtkStyleContext* style = gtk_widget_get_style_context(GetWidget(aWidgetType));
   gtk_style_context_save(style);
   MOZ_ASSERT(!sStyleContextNeedsRestore);
@@ -317,16 +312,30 @@ GetWidgetStyleInternal(WidgetNodeType aN
       return GetWidgetStyleWithClass(MOZ_GTK_RADIOBUTTON_CONTAINER,
                                      GTK_STYLE_CLASS_RADIO);
     case MOZ_GTK_CHECKBUTTON:
       return GetWidgetStyleWithClass(MOZ_GTK_CHECKBUTTON_CONTAINER,
                                      GTK_STYLE_CLASS_CHECK);
     case MOZ_GTK_PROGRESS_TROUGH:
       return GetWidgetStyleWithClass(MOZ_GTK_PROGRESSBAR,
                                      GTK_STYLE_CLASS_TROUGH);
+    case MOZ_GTK_TOOLTIP: {
+      GtkStyleContext* style = sStyleStorage[aNodeType];
+      if (style)
+        return style;
+
+      // The tooltip style class is added first in CreateTooltipWidget() so
+      // that gtk_widget_path_append_for_widget() in CreateStyleForWidget()
+      // will find it.
+      GtkWidget* tooltipWindow = CreateTooltipWidget();
+      style = CreateStyleForWidget(tooltipWindow, nullptr);
+      gtk_widget_destroy(tooltipWindow); // Release GtkWindow self-reference.
+      sStyleStorage[aNodeType] = style;
+      return style;
+    }
     default:
       GtkWidget* widget = GetWidget(aNodeType);
       MOZ_ASSERT(widget);
       return gtk_widget_get_style_context(widget);
   }
 }
 
 void