diff --git a/browser/base/content/test/static/browser_parsable_css.js b/browser/base/content/test/static/browser_parsable_css.js
index 6ff480fddc13..0cfd066cdc82 100644
--- a/browser/base/content/test/static/browser_parsable_css.js
+++ b/browser/base/content/test/static/browser_parsable_css.js
@@ -331,8 +331,7 @@ function processCSSRules(container) {
       continue;
     }
     if (rule.cssRules) {
-      processCSSRules(rule); // @supports, @media, @layer (block), @keyframes
-      continue;
+      processCSSRules(rule); // @supports, @media, @layer (block), @keyframes, style rules with nested rules.
     }
     if (!rule.style) {
       continue; // @layer (statement), @font-feature-values, @counter-style
diff --git a/dom/webidl/CSSGroupingRule.webidl b/dom/webidl/CSSGroupingRule.webidl
index 32c33814e06a..1cb6d49cb989 100644
--- a/dom/webidl/CSSGroupingRule.webidl
+++ b/dom/webidl/CSSGroupingRule.webidl
@@ -10,7 +10,7 @@
 // https://drafts.csswg.org/cssom/#cssgroupingrule
 [Exposed=Window]
 interface CSSGroupingRule : CSSRule {
-  [SameObject] readonly attribute CSSRuleList? cssRules;
+  [SameObject] readonly attribute CSSRuleList cssRules;
   [Throws]
   unsigned long insertRule(UTF8String rule, optional unsigned long index = 0);
   [Throws]
diff --git a/dom/webidl/CSSStyleRule.webidl b/dom/webidl/CSSStyleRule.webidl
index 739bd958bb20..1e65d1a2233a 100644
--- a/dom/webidl/CSSStyleRule.webidl
+++ b/dom/webidl/CSSStyleRule.webidl
@@ -12,4 +12,11 @@
 interface CSSStyleRule : CSSRule {
   attribute UTF8String selectorText;
   [SameObject, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;
+
+  // https://drafts.csswg.org/css-nesting/#cssom-style
+  // FIXME: Ideally CSSStyleRule should inherit from CSSGroupingRule instead,
+  // see https://github.com/w3c/csswg-drafts/issues/8940
+  [Pref="layout.css.nesting.enabled", SameObject] readonly attribute CSSRuleList cssRules;
+  [Pref="layout.css.nesting.enabled", Throws] unsigned long insertRule(UTF8String rule, optional unsigned long index = 0);
+  [Pref="layout.css.nesting.enabled", Throws] undefined deleteRule(unsigned long index);
 };
diff --git a/layout/inspector/ServoStyleRuleMap.cpp b/layout/inspector/ServoStyleRuleMap.cpp
index c7daae234d7a..2285ec8298b6 100644
--- a/layout/inspector/ServoStyleRuleMap.cpp
+++ b/layout/inspector/ServoStyleRuleMap.cpp
@@ -37,7 +37,7 @@ void ServoStyleRuleMap::EnsureTable(ShadowRoot& aShadowRoot) {
   for (auto index : IntegerRange(aShadowRoot.SheetCount())) {
     FillTableFromStyleSheet(*aShadowRoot.SheetAt(index));
   }
-  for (auto& sheet : aShadowRoot.AdoptedStyleSheets()) {
+  for (const auto& sheet : aShadowRoot.AdoptedStyleSheets()) {
     FillTableFromStyleSheet(*sheet);
   }
 }
@@ -80,18 +80,14 @@ void ServoStyleRuleMap::RuleRemoved(StyleSheet& aStyleSheet,
   }
 
   switch (aStyleRule.Type()) {
-    case StyleCssRuleType::Style: {
-      auto& rule = static_cast<CSSStyleRule&>(aStyleRule);
-      mTable.Remove(rule.Raw());
-      break;
-    }
+    case StyleCssRuleType::Style:
     case StyleCssRuleType::Import:
     case StyleCssRuleType::Media:
     case StyleCssRuleType::Supports:
     case StyleCssRuleType::LayerBlock:
     case StyleCssRuleType::Container:
     case StyleCssRuleType::Document: {
-      // See the comment in StyleSheetRemoved.
+      // See the comment in SheetRemoved.
       mTable.Clear();
       break;
     }
@@ -121,7 +117,7 @@ void ServoStyleRuleMap::FillTableFromRule(css::Rule& aRule) {
     case StyleCssRuleType::Style: {
       auto& rule = static_cast<CSSStyleRule&>(aRule);
       mTable.InsertOrUpdate(rule.Raw(), &rule);
-      break;
+      [[fallthrough]];
     }
     case StyleCssRuleType::LayerBlock:
     case StyleCssRuleType::Media:
@@ -129,9 +125,7 @@ void ServoStyleRuleMap::FillTableFromRule(css::Rule& aRule) {
     case StyleCssRuleType::Container:
     case StyleCssRuleType::Document: {
       auto& rule = static_cast<css::GroupRule&>(aRule);
-      if (ServoCSSRuleList* ruleList = rule.GetCssRules()) {
-        FillTableFromRuleList(*ruleList);
-      }
+      FillTableFromRuleList(*rule.CssRules());
       break;
     }
     case StyleCssRuleType::Import: {
diff --git a/layout/inspector/ServoStyleRuleMap.h b/layout/inspector/ServoStyleRuleMap.h
index 65399c043243..09d5dae260af 100644
--- a/layout/inspector/ServoStyleRuleMap.h
+++ b/layout/inspector/ServoStyleRuleMap.h
@@ -55,9 +55,8 @@ class ServoStyleRuleMap {
   void FillTableFromRuleList(ServoCSSRuleList&);
   void FillTableFromStyleSheet(StyleSheet&);
 
-  typedef nsTHashMap<nsPtrHashKey<const StyleLockedStyleRule>,
-                     WeakPtr<dom::CSSStyleRule>>
-      Hashtable;
+  using Hashtable = nsTHashMap<nsPtrHashKey<const StyleLockedStyleRule>,
+                               WeakPtr<dom::CSSStyleRule>>;
   Hashtable mTable;
 };
 
diff --git a/layout/style/CSSContainerRule.cpp b/layout/style/CSSContainerRule.cpp
index a92874f45419..4d798c0dda13 100644
--- a/layout/style/CSSContainerRule.cpp
+++ b/layout/style/CSSContainerRule.cpp
@@ -17,8 +17,7 @@ namespace mozilla::dom {
 CSSContainerRule::CSSContainerRule(RefPtr<StyleContainerRule> aRawRule,
                                    StyleSheet* aSheet, css::Rule* aParentRule,
                                    uint32_t aLine, uint32_t aColumn)
-    : css::ConditionRule(Servo_ContainerRule_GetRules(aRawRule).Consume(),
-                         aSheet, aParentRule, aLine, aColumn),
+    : css::ConditionRule(aSheet, aParentRule, aLine, aColumn),
       mRawRule(std::move(aRawRule)) {}
 
 CSSContainerRule::~CSSContainerRule() = default;
@@ -42,6 +41,10 @@ void CSSContainerRule::List(FILE* out, int32_t aIndent) const {
 }
 #endif
 
+already_AddRefed<StyleLockedCssRules> CSSContainerRule::GetOrCreateRawRules() {
+  return Servo_ContainerRule_GetRules(mRawRule).Consume();
+}
+
 StyleCssRuleType CSSContainerRule::Type() const {
   return StyleCssRuleType::Container;
 }
@@ -70,9 +73,7 @@ Element* CSSContainerRule::QueryContainerFor(const Element& aElement) const {
 
 void CSSContainerRule::SetRawAfterClone(RefPtr<StyleContainerRule> aRaw) {
   mRawRule = std::move(aRaw);
-
-  css::ConditionRule::SetRawAfterClone(
-      Servo_ContainerRule_GetRules(mRawRule).Consume());
+  css::ConditionRule::DidSetRawAfterClone();
 }
 
 /* virtual */
diff --git a/layout/style/CSSContainerRule.h b/layout/style/CSSContainerRule.h
index abe00e12d365..683a17604ec1 100644
--- a/layout/style/CSSContainerRule.h
+++ b/layout/style/CSSContainerRule.h
@@ -26,6 +26,8 @@ class CSSContainerRule final : public css::ConditionRule {
   StyleContainerRule* Raw() const { return mRawRule; }
   void SetRawAfterClone(RefPtr<StyleContainerRule>);
 
+  already_AddRefed<StyleLockedCssRules> GetOrCreateRawRules() final;
+
   // WebIDL interface
   StyleCssRuleType Type() const override;
   // WebIDL interface
diff --git a/layout/style/CSSLayerBlockRule.cpp b/layout/style/CSSLayerBlockRule.cpp
index acc57949e3c2..23453fb3af41 100644
--- a/layout/style/CSSLayerBlockRule.cpp
+++ b/layout/style/CSSLayerBlockRule.cpp
@@ -13,16 +13,13 @@ namespace mozilla::dom {
 CSSLayerBlockRule::CSSLayerBlockRule(RefPtr<StyleLayerBlockRule> aRawRule,
                                      StyleSheet* aSheet, css::Rule* aParentRule,
                                      uint32_t aLine, uint32_t aColumn)
-    : css::GroupRule(Servo_LayerBlockRule_GetRules(aRawRule).Consume(), aSheet,
-                     aParentRule, aLine, aColumn),
+    : css::GroupRule(aSheet, aParentRule, aLine, aColumn),
       mRawRule(std::move(aRawRule)) {}
 
-NS_IMPL_ADDREF_INHERITED(CSSLayerBlockRule, GroupRule)
-NS_IMPL_RELEASE_INHERITED(CSSLayerBlockRule, GroupRule)
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED_0(CSSLayerBlockRule,
+                                               css::GroupRule)
 
 // QueryInterface implementation for SupportsRule
-NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(CSSLayerBlockRule)
-NS_INTERFACE_MAP_END_INHERITING(GroupRule)
 
 #ifdef DEBUG
 void CSSLayerBlockRule::List(FILE* out, int32_t aIndent) const {
@@ -39,10 +36,13 @@ StyleCssRuleType CSSLayerBlockRule::Type() const {
   return StyleCssRuleType::LayerBlock;
 }
 
+already_AddRefed<StyleLockedCssRules> CSSLayerBlockRule::GetOrCreateRawRules() {
+  return Servo_LayerBlockRule_GetRules(mRawRule).Consume();
+}
+
 void CSSLayerBlockRule::SetRawAfterClone(RefPtr<StyleLayerBlockRule> aRaw) {
   mRawRule = std::move(aRaw);
-  css::GroupRule::SetRawAfterClone(
-      Servo_LayerBlockRule_GetRules(mRawRule).Consume());
+  css::GroupRule::DidSetRawAfterClone();
 }
 
 void CSSLayerBlockRule::GetCssText(nsACString& aCssText) const {
diff --git a/layout/style/CSSLayerBlockRule.h b/layout/style/CSSLayerBlockRule.h
index 7679468c15d7..fbba832eef1f 100644
--- a/layout/style/CSSLayerBlockRule.h
+++ b/layout/style/CSSLayerBlockRule.h
@@ -26,6 +26,8 @@ class CSSLayerBlockRule final : public css::GroupRule {
   StyleLayerBlockRule* Raw() const { return mRawRule; }
   void SetRawAfterClone(RefPtr<StyleLayerBlockRule>);
 
+  already_AddRefed<StyleLockedCssRules> GetOrCreateRawRules() final;
+
   // WebIDL interface
   StyleCssRuleType Type() const final;
   void GetCssText(nsACString& aCssText) const final;
diff --git a/layout/style/CSSMediaRule.cpp b/layout/style/CSSMediaRule.cpp
index f41d5ddbf5b0..75b4ce4571be 100644
--- a/layout/style/CSSMediaRule.cpp
+++ b/layout/style/CSSMediaRule.cpp
@@ -15,8 +15,7 @@ namespace mozilla::dom {
 CSSMediaRule::CSSMediaRule(RefPtr<StyleMediaRule> aRawRule, StyleSheet* aSheet,
                            css::Rule* aParentRule, uint32_t aLine,
                            uint32_t aColumn)
-    : ConditionRule(Servo_MediaRule_GetRules(aRawRule).Consume(), aSheet,
-                    aParentRule, aLine, aColumn),
+    : ConditionRule(aSheet, aParentRule, aLine, aColumn),
       mRawRule(std::move(aRawRule)) {}
 
 CSSMediaRule::~CSSMediaRule() {
@@ -25,12 +24,7 @@ CSSMediaRule::~CSSMediaRule() {
   }
 }
 
-NS_IMPL_ADDREF_INHERITED(CSSMediaRule, css::ConditionRule)
-NS_IMPL_RELEASE_INHERITED(CSSMediaRule, css::ConditionRule)
-
-// QueryInterface implementation for MediaRule
-NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(CSSMediaRule)
-NS_INTERFACE_MAP_END_INHERITING(css::ConditionRule)
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED_0(CSSMediaRule, css::ConditionRule)
 
 NS_IMPL_CYCLE_COLLECTION_CLASS(CSSMediaRule)
 
@@ -47,6 +41,10 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(CSSMediaRule,
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mMediaList)
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
 
+bool CSSMediaRule::IsCCLeaf() const {
+  return ConditionRule::IsCCLeaf() && !mMediaList;
+}
+
 /* virtual */
 void CSSMediaRule::DropSheetReference() {
   if (mMediaList) {
@@ -62,8 +60,11 @@ void CSSMediaRule::SetRawAfterClone(RefPtr<StyleMediaRule> aRaw) {
     mMediaList->SetStyleSheet(nullptr);
     mMediaList->SetStyleSheet(GetStyleSheet());
   }
-  css::ConditionRule::SetRawAfterClone(
-      Servo_MediaRule_GetRules(mRawRule).Consume());
+  css::ConditionRule::DidSetRawAfterClone();
+}
+
+already_AddRefed<StyleLockedCssRules> CSSMediaRule::GetOrCreateRawRules() {
+  return Servo_MediaRule_GetRules(mRawRule).Consume();
 }
 
 StyleCssRuleType CSSMediaRule::Type() const { return StyleCssRuleType::Media; }
diff --git a/layout/style/CSSMediaRule.h b/layout/style/CSSMediaRule.h
index a1c430212795..d5904ca5cb7c 100644
--- a/layout/style/CSSMediaRule.h
+++ b/layout/style/CSSMediaRule.h
@@ -28,6 +28,8 @@ class CSSMediaRule final : public css::ConditionRule {
 
   StyleMediaRule* Raw() const { return mRawRule; }
   void SetRawAfterClone(RefPtr<StyleMediaRule>);
+  already_AddRefed<StyleLockedCssRules> GetOrCreateRawRules() final;
+  bool IsCCLeaf() const override;
 
   // WebIDL interface
   StyleCssRuleType Type() const override;
diff --git a/layout/style/CSSMozDocumentRule.cpp b/layout/style/CSSMozDocumentRule.cpp
index f06fe38207e7..b59a66d85640 100644
--- a/layout/style/CSSMozDocumentRule.cpp
+++ b/layout/style/CSSMozDocumentRule.cpp
@@ -88,8 +88,7 @@ CSSMozDocumentRule::CSSMozDocumentRule(RefPtr<StyleDocumentRule> aRawRule,
                                        StyleSheet* aSheet,
                                        css::Rule* aParentRule, uint32_t aLine,
                                        uint32_t aColumn)
-    : css::ConditionRule(Servo_DocumentRule_GetRules(aRawRule).Consume(),
-                         aSheet, aParentRule, aLine, aColumn),
+    : css::ConditionRule(aSheet, aParentRule, aLine, aColumn),
       mRawRule(std::move(aRawRule)) {}
 
 NS_IMPL_ADDREF_INHERITED(CSSMozDocumentRule, css::ConditionRule)
@@ -113,8 +112,12 @@ void CSSMozDocumentRule::List(FILE* out, int32_t aIndent) const {
 
 void CSSMozDocumentRule::SetRawAfterClone(RefPtr<StyleDocumentRule> aRaw) {
   mRawRule = std::move(aRaw);
-  css::ConditionRule::SetRawAfterClone(
-      Servo_DocumentRule_GetRules(mRawRule).Consume());
+  css::ConditionRule::DidSetRawAfterClone();
+}
+
+already_AddRefed<StyleLockedCssRules>
+CSSMozDocumentRule::GetOrCreateRawRules() {
+  return Servo_DocumentRule_GetRules(mRawRule).Consume();
 }
 
 StyleCssRuleType CSSMozDocumentRule::Type() const {
diff --git a/layout/style/CSSMozDocumentRule.h b/layout/style/CSSMozDocumentRule.h
index b75da3f7b820..0c1b0ab0227c 100644
--- a/layout/style/CSSMozDocumentRule.h
+++ b/layout/style/CSSMozDocumentRule.h
@@ -11,8 +11,7 @@
 #include "mozilla/css/DocumentMatchingFunction.h"
 #include "mozilla/ServoBindingTypes.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class CSSMozDocumentRule final : public css::ConditionRule {
  public:
@@ -31,6 +30,7 @@ class CSSMozDocumentRule final : public css::ConditionRule {
 
   StyleDocumentRule* Raw() const { return mRawRule; }
   void SetRawAfterClone(RefPtr<StyleDocumentRule>);
+  already_AddRefed<StyleLockedCssRules> GetOrCreateRawRules() final;
 
   // WebIDL interface
   StyleCssRuleType Type() const final;
@@ -48,7 +48,6 @@ class CSSMozDocumentRule final : public css::ConditionRule {
   RefPtr<StyleDocumentRule> mRawRule;
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_CSSMozDocumentRule_h
diff --git a/layout/style/CSSStyleRule.cpp b/layout/style/CSSStyleRule.cpp
index 966725371f5a..fd57d09162bc 100644
--- a/layout/style/CSSStyleRule.cpp
+++ b/layout/style/CSSStyleRule.cpp
@@ -70,6 +70,11 @@ DeclarationBlock* CSSStyleRuleDeclaration::GetOrCreateCSSDeclaration(
 void CSSStyleRule::SetRawAfterClone(RefPtr<StyleLockedStyleRule> aRaw) {
   mRawRule = std::move(aRaw);
   mDecls.SetRawAfterClone(Servo_StyleRule_GetStyle(mRawRule).Consume());
+  GroupRule::DidSetRawAfterClone();
+}
+
+already_AddRefed<StyleLockedCssRules> CSSStyleRule::GetOrCreateRawRules() {
+  return Servo_StyleRule_EnsureRules(mRawRule, IsReadOnly()).Consume();
 }
 
 void CSSStyleRuleDeclaration::SetRawAfterClone(
@@ -110,15 +115,15 @@ CSSStyleRuleDeclaration::GetParsingEnvironment(
 CSSStyleRule::CSSStyleRule(already_AddRefed<StyleLockedStyleRule> aRawRule,
                            StyleSheet* aSheet, css::Rule* aParentRule,
                            uint32_t aLine, uint32_t aColumn)
-    : css::Rule(aSheet, aParentRule, aLine, aColumn),
+    : GroupRule(aSheet, aParentRule, aLine, aColumn),
       mRawRule(aRawRule),
       mDecls(Servo_StyleRule_GetStyle(mRawRule).Consume()) {}
 
-NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED_0(CSSStyleRule, css::Rule)
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED_0(CSSStyleRule, GroupRule)
 
 NS_IMPL_CYCLE_COLLECTION_CLASS(CSSStyleRule)
 
-NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN_INHERITED(CSSStyleRule, css::Rule)
+NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN_INHERITED(CSSStyleRule, GroupRule)
   // Keep this in sync with IsCCLeaf.
 
   // Trace the wrapper for our declaration.  This just expands out
@@ -135,17 +140,16 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(CSSStyleRule)
   // Note that this has to happen before unlinking css::Rule.
   tmp->UnlinkDeclarationWrapper(tmp->mDecls);
   NS_IMPL_CYCLE_COLLECTION_UNLINK_WEAK_PTR
-NS_IMPL_CYCLE_COLLECTION_UNLINK_END_INHERITED(css::Rule)
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END_INHERITED(GroupRule)
 
-NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(CSSStyleRule, css::Rule)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(CSSStyleRule, GroupRule)
   // Keep this in sync with IsCCLeaf.
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
 
 bool CSSStyleRule::IsCCLeaf() const {
-  if (!Rule::IsCCLeaf()) {
+  if (!GroupRule::IsCCLeaf()) {
     return false;
   }
-
   return !mDecls.PreservingWrapper();
 }
 
diff --git a/layout/style/CSSStyleRule.h b/layout/style/CSSStyleRule.h
index 70f20a69ebf4..16157a2c2e21 100644
--- a/layout/style/CSSStyleRule.h
+++ b/layout/style/CSSStyleRule.h
@@ -7,7 +7,7 @@
 #ifndef mozilla_CSSStyleRule_h
 #define mozilla_CSSStyleRule_h
 
-#include "mozilla/css/Rule.h"
+#include "mozilla/css/GroupRule.h"
 #include "mozilla/ServoBindingTypes.h"
 #include "mozilla/NotNull.h"
 #include "mozilla/WeakPtr.h"
@@ -55,7 +55,7 @@ class CSSStyleRuleDeclaration final : public nsDOMCSSDeclaration {
   RefPtr<DeclarationBlock> mDecls;
 };
 
-class CSSStyleRule final : public css::Rule, public SupportsWeakPtr {
+class CSSStyleRule final : public css::GroupRule, public SupportsWeakPtr {
  public:
   CSSStyleRule(already_AddRefed<StyleLockedStyleRule> aRawRule,
                StyleSheet* aSheet, css::Rule* aParentRule, uint32_t aLine,
@@ -63,7 +63,7 @@ class CSSStyleRule final : public css::Rule, public SupportsWeakPtr {
 
   NS_DECL_ISUPPORTS_INHERITED
   NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_INHERITED(CSSStyleRule,
-                                                         css::Rule)
+                                                         css::GroupRule)
   bool IsCCLeaf() const final MOZ_MUST_OVERRIDE;
 
   uint32_t GetSelectorCount() const;
@@ -84,6 +84,7 @@ class CSSStyleRule final : public css::Rule, public SupportsWeakPtr {
 
   StyleLockedStyleRule* Raw() const { return mRawRule; }
   void SetRawAfterClone(RefPtr<StyleLockedStyleRule>);
+  already_AddRefed<StyleLockedCssRules> GetOrCreateRawRules() final;
 
   // Methods of mozilla::css::Rule
   size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const final;
diff --git a/layout/style/CSSSupportsRule.cpp b/layout/style/CSSSupportsRule.cpp
index 1f1bbae3799c..e848258ed886 100644
--- a/layout/style/CSSSupportsRule.cpp
+++ b/layout/style/CSSSupportsRule.cpp
@@ -17,8 +17,7 @@ namespace mozilla::dom {
 CSSSupportsRule::CSSSupportsRule(RefPtr<StyleSupportsRule> aRawRule,
                                  StyleSheet* aSheet, css::Rule* aParentRule,
                                  uint32_t aLine, uint32_t aColumn)
-    : css::ConditionRule(Servo_SupportsRule_GetRules(aRawRule).Consume(),
-                         aSheet, aParentRule, aLine, aColumn),
+    : css::ConditionRule(aSheet, aParentRule, aLine, aColumn),
       mRawRule(std::move(aRawRule)) {}
 
 NS_IMPL_ADDREF_INHERITED(CSSSupportsRule, ConditionRule)
@@ -55,9 +54,11 @@ void CSSSupportsRule::GetCssText(nsACString& aCssText) const {
 
 void CSSSupportsRule::SetRawAfterClone(RefPtr<StyleSupportsRule> aRaw) {
   mRawRule = std::move(aRaw);
+  css::ConditionRule::DidSetRawAfterClone();
+}
 
-  css::ConditionRule::SetRawAfterClone(
-      Servo_SupportsRule_GetRules(mRawRule).Consume());
+already_AddRefed<StyleLockedCssRules> CSSSupportsRule::GetOrCreateRawRules() {
+  return Servo_SupportsRule_GetRules(mRawRule).Consume();
 }
 
 /* virtual */
diff --git a/layout/style/CSSSupportsRule.h b/layout/style/CSSSupportsRule.h
index b6c40c6b3eac..58b622c96fdd 100644
--- a/layout/style/CSSSupportsRule.h
+++ b/layout/style/CSSSupportsRule.h
@@ -10,10 +10,9 @@
 #include "mozilla/css/GroupRule.h"
 #include "mozilla/ServoBindingTypes.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
-class CSSSupportsRule : public css::ConditionRule {
+class CSSSupportsRule final : public css::ConditionRule {
  public:
   CSSSupportsRule(RefPtr<StyleSupportsRule> aRawRule, StyleSheet* aSheet,
                   css::Rule* aParentRule, uint32_t aLine, uint32_t aColumn);
@@ -31,6 +30,7 @@ class CSSSupportsRule : public css::ConditionRule {
   StyleCssRuleType Type() const final;
   void GetCssText(nsACString& aCssText) const final;
   void GetConditionText(nsACString& aConditionText) final;
+  already_AddRefed<StyleLockedCssRules> GetOrCreateRawRules() final;
 
   size_t SizeOfIncludingThis(MallocSizeOf) const override;
   JSObject* WrapObject(JSContext* aCx,
@@ -42,7 +42,6 @@ class CSSSupportsRule : public css::ConditionRule {
   RefPtr<StyleSupportsRule> mRawRule;
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_CSSSupportsRule_h
diff --git a/layout/style/GroupRule.cpp b/layout/style/GroupRule.cpp
index 018be3109efc..ff3cd11f8c3c 100644
--- a/layout/style/GroupRule.cpp
+++ b/layout/style/GroupRule.cpp
@@ -19,11 +19,9 @@ using namespace mozilla::dom;
 
 namespace mozilla::css {
 
-GroupRule::GroupRule(already_AddRefed<StyleLockedCssRules> aRules,
-                     StyleSheet* aSheet, Rule* aParentRule,
+GroupRule::GroupRule(StyleSheet* aSheet, Rule* aParentRule,
                      uint32_t aLineNumber, uint32_t aColumnNumber)
-    : Rule(aSheet, aParentRule, aLineNumber, aColumnNumber),
-      mRuleList(new ServoCSSRuleList(std::move(aRules), aSheet, this)) {}
+    : Rule(aSheet, aParentRule, aLineNumber, aColumnNumber) {}
 
 GroupRule::~GroupRule() {
   MOZ_ASSERT(!mSheet, "SetStyleSheet should have been called");
@@ -39,8 +37,20 @@ NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(GroupRule)
 NS_INTERFACE_MAP_END_INHERITING(Rule)
 
 bool GroupRule::IsCCLeaf() const {
-  // Let's not worry for now about sorting out whether we're a leaf or not.
-  return false;
+  if (!Rule::IsCCLeaf()) {
+    return false;
+  }
+  return !mRuleList;
+}
+
+ServoCSSRuleList* GroupRule::CssRules() {
+  if (!mRuleList) {
+    // Lazily create the rule list since most style rules won't have child
+    // rules.
+    mRuleList =
+        new ServoCSSRuleList(GetOrCreateRawRules(), GetStyleSheet(), this);
+  }
+  return mRuleList;
 }
 
 NS_IMPL_CYCLE_COLLECTION_CLASS(GroupRule)
diff --git a/layout/style/GroupRule.h b/layout/style/GroupRule.h
index 5994436a7b06..ca3bd58e083f 100644
--- a/layout/style/GroupRule.h
+++ b/layout/style/GroupRule.h
@@ -29,37 +29,31 @@ class CSSRuleList;
 
 namespace css {
 
-// inherits from Rule so it can be shared between
-// MediaRule and DocumentRule
+// Inherits from Rule so it can be shared between MediaRule and DocumentRule
 class GroupRule : public Rule {
  protected:
-  GroupRule(already_AddRefed<StyleLockedCssRules> aRules, StyleSheet* aSheet,
-            Rule* aParentRule, uint32_t aLineNumber, uint32_t aColumnNumber);
-  GroupRule(const GroupRule& aCopy) = delete;
+  GroupRule(StyleSheet* aSheet, Rule* aParentRule, uint32_t aLineNumber,
+            uint32_t aColumnNumber);
   virtual ~GroupRule();
+  virtual already_AddRefed<StyleLockedCssRules> GetOrCreateRawRules() = 0;
 
  public:
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(GroupRule, Rule)
   NS_DECL_ISUPPORTS_INHERITED
-  virtual bool IsCCLeaf() const override;
+
+  GroupRule(const GroupRule&) = delete;
+  bool IsCCLeaf() const override;
 
 #ifdef DEBUG
   void List(FILE* out = stdout, int32_t aIndent = 0) const override;
 #endif
   void DropSheetReference() override;
+  uint32_t StyleRuleCount() { return CssRules()->Length(); }
+  Rule* GetStyleRuleAt(int32_t aIndex) { return CssRules()->GetRule(aIndex); }
 
- public:
-  int32_t StyleRuleCount() const { return mRuleList ? mRuleList->Length() : 0; }
-
-  Rule* GetStyleRuleAt(int32_t aIndex) const {
-    return mRuleList ? mRuleList->GetRule(aIndex) : nullptr;
-  }
-
-  void SetRawAfterClone(RefPtr<StyleLockedCssRules> aRules) {
+  void DidSetRawAfterClone() {
     if (mRuleList) {
-      mRuleList->SetRawAfterClone(std::move(aRules));
-    } else {
-      MOZ_ASSERT(!aRules, "Can't move from having no rules to having rules");
+      mRuleList->SetRawAfterClone(GetOrCreateRawRules());
     }
   }
 
@@ -68,10 +62,7 @@ class GroupRule : public Rule {
    * WillDirty() on the parent stylesheet.
    */
   nsresult DeleteStyleRuleAt(uint32_t aIndex) {
-    if (!mRuleList) {
-      return NS_OK;
-    }
-    return mRuleList->DeleteRule(aIndex);
+    return CssRules()->DeleteRule(aIndex);
   }
 
   // non-virtual -- it is only called by subclasses
@@ -79,7 +70,7 @@ class GroupRule : public Rule {
   size_t SizeOfIncludingThis(MallocSizeOf) const override = 0;
 
   // WebIDL API
-  ServoCSSRuleList* GetCssRules() { return mRuleList; }
+  ServoCSSRuleList* CssRules();
   uint32_t InsertRule(const nsACString& aRule, uint32_t aIndex,
                       ErrorResult& aRv);
   void DeleteRule(uint32_t aIndex, ErrorResult& aRv);
diff --git a/layout/style/Rule.cpp b/layout/style/Rule.cpp
index a107dc3f01b4..0a7de4278905 100644
--- a/layout/style/Rule.cpp
+++ b/layout/style/Rule.cpp
@@ -101,6 +101,7 @@ void Rule::AssertParentRuleType() {
   if (mParentRule) {
     auto type = mParentRule->Type();
     MOZ_ASSERT(type == StyleCssRuleType::Media ||
+               type == StyleCssRuleType::Style ||
                type == StyleCssRuleType::Document ||
                type == StyleCssRuleType::Supports ||
                type == StyleCssRuleType::Keyframes ||
diff --git a/layout/style/StyleSheet.cpp b/layout/style/StyleSheet.cpp
index 5f639cada72d..53cefeb5a01c 100644
--- a/layout/style/StyleSheet.cpp
+++ b/layout/style/StyleSheet.cpp
@@ -1466,7 +1466,7 @@ nsresult StyleSheet::InsertRuleIntoGroupInternal(const nsACString& aRule,
                                                  uint32_t aIndex) {
   MOZ_ASSERT(!IsReadOnly());
 
-  ServoCSSRuleList* rules = aGroup->GetCssRules();
+  ServoCSSRuleList* rules = aGroup->CssRules();
   MOZ_ASSERT(rules && rules->GetParentRule() == aGroup);
   return rules->InsertRule(aRule, aIndex);
 }
diff --git a/servo/ports/geckolib/glue.rs b/servo/ports/geckolib/glue.rs
index 0238c632a023..d5eb7cf50ea6 100644
--- a/servo/ports/geckolib/glue.rs
+++ b/servo/ports/geckolib/glue.rs
@@ -2270,6 +2270,25 @@ impl_basic_rule_funcs! { (Style, StyleRule, Locked<StyleRule>),
     changed: Servo_StyleSet_StyleRuleChanged,
 }
 
+#[no_mangle]
+pub extern "C" fn Servo_StyleRule_EnsureRules(rule: &LockedStyleRule, read_only: bool) -> Strong<LockedCssRules> {
+    let global_style_data = &*GLOBAL_STYLE_DATA;
+    let lock = &global_style_data.shared_lock;
+    if read_only {
+        let guard = lock.read();
+        if let Some(ref rules) = rule.read_with(&guard).rules {
+            return rules.clone().into();
+        }
+        return CssRules::new(vec![], lock).into();
+    }
+    let mut guard = lock.write();
+    rule.write_with(&mut guard)
+        .rules
+        .get_or_insert_with(|| CssRules::new(vec![], lock))
+        .clone()
+        .into()
+}
+
 impl_basic_rule_funcs! { (Import, ImportRule, Locked<ImportRule>),
     getter: Servo_CssRules_GetImportRuleAt,
     debug: Servo_ImportRule_Debug,
diff --git a/testing/web-platform/meta/css/css-nesting/cssom.html.ini b/testing/web-platform/meta/css/css-nesting/cssom.html.ini
index cf18f78006c5..0833d8ad04d7 100644
--- a/testing/web-platform/meta/css/css-nesting/cssom.html.ini
+++ b/testing/web-platform/meta/css/css-nesting/cssom.html.ini
@@ -1,28 +1,4 @@
 [cssom.html]
-  [Simple CSSOM manipulation of subrules]
-    expected: FAIL
-
-  [Simple CSSOM manipulation of subrules 1]
-    expected: FAIL
-
-  [Simple CSSOM manipulation of subrules 2]
-    expected: FAIL
-
-  [Simple CSSOM manipulation of subrules 3]
-    expected: FAIL
-
-  [Simple CSSOM manipulation of subrules 4]
-    expected: FAIL
-
-  [Simple CSSOM manipulation of subrules 5]
-    expected: FAIL
-
-  [Simple CSSOM manipulation of subrules 6]
-    expected: FAIL
-
-  [Simple CSSOM manipulation of subrules 7]
-    expected: FAIL
-
   [Simple CSSOM manipulation of subrules 9]
     expected: FAIL