--- a/widget/gtk/IMContextWrapper.cpp
+++ b/widget/gtk/IMContextWrapper.cpp
@@ -266,31 +266,38 @@ class SelectionStyleProvider final {
     nsAutoCString style(":selected{");
     // FYI: LookAndFeel always returns selection colors of GtkTextView.
     nscolor selectionForegroundColor;
     if (NS_SUCCEEDED(
             LookAndFeel::GetColor(LookAndFeel::eColorID_TextSelectForeground,
                                   &selectionForegroundColor))) {
       double alpha =
           static_cast<double>(NS_GET_A(selectionForegroundColor)) / 0xFF;
-      style.AppendPrintf("color:rgba(%u,%u,%u,%f);",
+      style.AppendPrintf("color:rgba(%u,%u,%u,",
                          NS_GET_R(selectionForegroundColor),
                          NS_GET_G(selectionForegroundColor),
-                         NS_GET_B(selectionForegroundColor), alpha);
+                         NS_GET_B(selectionForegroundColor));
+      // We can't use AppendPrintf here, because it does locale-specific
+      // formatting of floating-point values.
+      style.AppendFloat(alpha);
+      style.AppendPrintf(");");
+
     }
     nscolor selectionBackgroundColor;
     if (NS_SUCCEEDED(
             LookAndFeel::GetColor(LookAndFeel::eColorID_TextSelectBackground,
                                   &selectionBackgroundColor))) {
       double alpha =
           static_cast<double>(NS_GET_A(selectionBackgroundColor)) / 0xFF;
-      style.AppendPrintf("background-color:rgba(%u,%u,%u,%f);",
+      style.AppendPrintf("background-color:rgba(%u,%u,%u,",
                          NS_GET_R(selectionBackgroundColor),
                          NS_GET_G(selectionBackgroundColor),
-                         NS_GET_B(selectionBackgroundColor), alpha);
+                         NS_GET_B(selectionBackgroundColor));
+      style.AppendFloat(alpha);
+      style.AppendPrintf(");");
     }
     style.AppendLiteral("}");
     gtk_css_provider_load_from_data(mProvider, style.get(), -1, nullptr);
   }
 
  private:
   static SelectionStyleProvider* sInstance;
   static bool sHasShutDown;