diff --git a/layout/style/res/html.css b/layout/style/res/html.css
index 0ce8901252..93539c0890 100644
--- a/layout/style/res/html.css
+++ b/layout/style/res/html.css
@@ -893,3 +893,9 @@ rtc > rt {
 ruby, rb, rt, rtc {
   unicode-bidi: isolate;
 }
+
+/* Shadow DOM v1
+ * https://drafts.csswg.org/css-scoping/#slots-in-shadow-tree */
+slot {
+  display: contents;
+}
diff --git a/layout/style/res/ua.css b/layout/style/res/ua.css
index a8425d472a..ab51f67c53 100644
--- a/layout/style/res/ua.css
+++ b/layout/style/res/ua.css
@@ -474,9 +474,3 @@ div:-moz-native-anonymous.moz-custom-content-container {
   width: 100%;
   height: 100%;
 }
-
-/* Shadow DOM v1
- * https://drafts.csswg.org/css-scoping/#slots-in-shadow-tree */
-slot {
-  display: contents;
-}
\ No newline at end of file