--- a/layout/style/ServoCSSRuleList.h
+++ b/layout/style/ServoCSSRuleList.h
@@ -19,16 +19,17 @@ namespace css {
 class GroupRule;
 class Rule;
 } // namespace css
 
 class ServoCSSRuleList final : public dom::CSSRuleList
 {
 public:
   explicit ServoCSSRuleList(already_AddRefed<ServoCssRules> aRawRules);
+  css::GroupRule* GetParentRule() const { return mParentRule; }
   void SetParentRule(css::GroupRule* aParentRule);
   void SetStyleSheet(StyleSheet* aSheet);
 
   NS_DECL_ISUPPORTS_INHERITED
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(ServoCSSRuleList, dom::CSSRuleList)
 
   ServoStyleSheet* GetParentObject() final { return mStyleSheet; }
 

--- a/layout/style/ServoStyleSheet.cpp
+++ b/layout/style/ServoStyleSheet.cpp
@@ -5,16 +5,17 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/ServoStyleSheet.h"
 
 #include "mozilla/css/Rule.h"
 #include "mozilla/StyleBackendType.h"
 #include "mozilla/ServoBindings.h"
 #include "mozilla/ServoCSSRuleList.h"
+#include "mozilla/css/GroupRule.h"
 #include "mozilla/dom/CSSRuleList.h"
 
 #include "mozAutoDocUpdate.h"
 
 using namespace mozilla::dom;
 
 namespace mozilla {
 
@@ -197,9 +198,19 @@ ServoStyleSheet::DeleteRuleInternal(uint
   aRv = mRuleList->DeleteRule(aIndex);
   MOZ_ASSERT(!aRv.ErrorCodeIs(NS_ERROR_DOM_INDEX_SIZE_ERR),
              "IndexSizeError should have been handled earlier");
   if (!aRv.Failed() && mDocument) {
     mDocument->StyleRuleRemoved(this, rule);
   }
 }
 
+nsresult
+ServoStyleSheet::InsertRuleIntoGroupInternal(const nsAString& aRule,
+                                             css::GroupRule* aGroup,
+                                             uint32_t aIndex)
+{
+  auto rules = static_cast<ServoCSSRuleList*>(aGroup->CssRules());
+  MOZ_ASSERT(rules->GetParentRule() == aGroup);
+  return rules->InsertRule(aRule, aIndex);
+}
+
 } // namespace mozilla

--- a/layout/style/ServoStyleSheet.h
+++ b/layout/style/ServoStyleSheet.h
@@ -98,16 +98,19 @@ protected:
     return static_cast<ServoStyleSheetInner*>(mInner);
   }
 
   // Internal methods which do not have security check and completeness check.
   dom::CSSRuleList* GetCssRulesInternal(ErrorResult& aRv);
   uint32_t InsertRuleInternal(const nsAString& aRule,
                               uint32_t aIndex, ErrorResult& aRv);
   void DeleteRuleInternal(uint32_t aIndex, ErrorResult& aRv);
+  nsresult InsertRuleIntoGroupInternal(const nsAString& aRule,
+                                       css::GroupRule* aGroup,
+                                       uint32_t aIndex);
 
   void EnabledStateChangedInternal() {}
 
 private:
   ServoStyleSheet(const ServoStyleSheet& aCopy,
                   ServoStyleSheet* aParentToUse,
                   css::ImportRule* aOwnerRuleToUse,
                   nsIDocument* aDocumentToUse,

--- a/layout/style/StyleSheet.cpp
+++ b/layout/style/StyleSheet.cpp
@@ -468,18 +468,17 @@ StyleSheet::InsertRuleIntoGroup(const ns
   mozAutoDocUpdate updateBatch(mDocument, UPDATE_STYLE, true);
 
   WillDirty();
 
   nsresult result;
   if (IsGecko()) {
     result = AsGecko()->InsertRuleIntoGroupInternal(aRule, aGroup, aIndex);
   } else {
-    // TODO
-    result = NS_ERROR_NOT_IMPLEMENTED;
+    result = AsServo()->InsertRuleIntoGroupInternal(aRule, aGroup, aIndex);
   }
   NS_ENSURE_SUCCESS(result, result);
 
   DidDirty();
 
   if (mDocument) {
     mDocument->StyleRuleAdded(this, aGroup->GetStyleRuleAt(aIndex));
   }