diff --git a/chrome/nsChromeRegistry.cpp b/chrome/nsChromeRegistry.cpp
index 4ecb3bd2953c3..59519673019ed 100644
--- a/chrome/nsChromeRegistry.cpp
+++ b/chrome/nsChromeRegistry.cpp
@@ -17,7 +17,6 @@
 #include "nsString.h"
 #include "nsQueryObject.h"
 
-#include "mozilla/CSSStyleSheet.h"
 #include "mozilla/dom/URL.h"
 #include "nsIConsoleService.h"
 #include "nsIDocument.h"
@@ -30,12 +29,14 @@
 #include "nsIScriptError.h"
 #include "nsIWindowMediator.h"
 #include "nsIPrefService.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 
 nsChromeRegistry* nsChromeRegistry::gChromeRegistry;
 
 // DO NOT use namespace mozilla; it'll break due to a naming conflict between
 // mozilla::TextRange and a TextRange in OSX headers.
-using mozilla::CSSStyleSheet;
+using mozilla::StyleSheetHandle;
 using mozilla::dom::IsChromeURI;
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -401,19 +402,18 @@ nsresult nsChromeRegistry::RefreshWindow(nsPIDOMWindowOuter* aWindow)
   nsCOMPtr<nsIPresShell> shell = document->GetShell();
   if (shell) {
     // Reload only the chrome URL agent style sheets.
-    nsTArray<RefPtr<CSSStyleSheet>> agentSheets;
+    nsTArray<StyleSheetHandle::RefPtr> agentSheets;
     rv = shell->GetAgentStyleSheets(agentSheets);
     NS_ENSURE_SUCCESS(rv, rv);
 
-    nsTArray<RefPtr<CSSStyleSheet>> newAgentSheets;
-    for (CSSStyleSheet* sheet : agentSheets) {
+    nsTArray<StyleSheetHandle::RefPtr> newAgentSheets;
+    for (StyleSheetHandle sheet : agentSheets) {
       nsIURI* uri = sheet->GetSheetURI();
 
       if (IsChromeURI(uri)) {
         // Reload the sheet.
-        RefPtr<CSSStyleSheet> newSheet;
-        rv = document->LoadChromeSheetSync(uri, true,
-                                           getter_AddRefs(newSheet));
+        StyleSheetHandle::RefPtr newSheet;
+        rv = document->LoadChromeSheetSync(uri, true, &newSheet);
         if (NS_FAILED(rv)) return rv;
         if (newSheet) {
           rv = newAgentSheets.AppendElement(newSheet) ? NS_OK : NS_ERROR_FAILURE;
@@ -433,27 +433,27 @@ nsresult nsChromeRegistry::RefreshWindow(nsPIDOMWindowOuter* aWindow)
   int32_t count = document->GetNumberOfStyleSheets();
 
   // Build an array of style sheets we need to reload.
-  nsTArray<RefPtr<CSSStyleSheet>> oldSheets(count);
-  nsTArray<RefPtr<CSSStyleSheet>> newSheets(count);
+  nsTArray<StyleSheetHandle::RefPtr> oldSheets(count);
+  nsTArray<StyleSheetHandle::RefPtr> newSheets(count);
 
   // Iterate over the style sheets.
   for (int32_t i = 0; i < count; i++) {
     // Get the style sheet
-    CSSStyleSheet* styleSheet = document->GetStyleSheetAt(i);
+    StyleSheetHandle styleSheet = document->GetStyleSheetAt(i);
     oldSheets.AppendElement(styleSheet);
   }
 
   // Iterate over our old sheets and kick off a sync load of the new
   // sheet if and only if it's a chrome URL.
-  for (CSSStyleSheet* sheet : oldSheets) {
+  for (StyleSheetHandle sheet : oldSheets) {
     nsIURI* uri = sheet ? sheet->GetOriginalURI() : nullptr;
 
     if (uri && IsChromeURI(uri)) {
       // Reload the sheet.
-      RefPtr<CSSStyleSheet> newSheet;
+      StyleSheetHandle::RefPtr newSheet;
       // XXX what about chrome sheets that have a title or are disabled?  This
       // only works by sheer dumb luck.
-      document->LoadChromeSheetSync(uri, false, getter_AddRefs(newSheet));
+      document->LoadChromeSheetSync(uri, false, &newSheet);
       // Even if it's null, we put in in there.
       newSheets.AppendElement(newSheet);
     } else {
diff --git a/dom/base/ShadowRoot.cpp b/dom/base/ShadowRoot.cpp
index ed60e55ae27b9..120aeb34d2fb5 100644
--- a/dom/base/ShadowRoot.cpp
+++ b/dom/base/ShadowRoot.cpp
@@ -17,6 +17,8 @@
 #include "mozilla/dom/HTMLContentElement.h"
 #include "mozilla/dom/HTMLShadowElement.h"
 #include "nsXBLPrototypeBinding.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
@@ -130,7 +132,7 @@ ShadowRoot::StyleSheetChanged()
 }
 
 void
-ShadowRoot::InsertSheet(CSSStyleSheet* aSheet,
+ShadowRoot::InsertSheet(StyleSheetHandle aSheet,
                         nsIContent* aLinkingContent)
 {
   nsCOMPtr<nsIStyleSheetLinkingElement>
@@ -148,8 +150,8 @@ ShadowRoot::InsertSheet(CSSStyleSheet* aSheet,
       break;
     }
 
-    nsINode* sheetOwnerNode = mProtoBinding->StyleSheetAt(i)->GetOwnerNode();
-    if (nsContentUtils::PositionIsBefore(aLinkingContent, sheetOwnerNode)) {
+    nsINode* sheetOwningNode = mProtoBinding->StyleSheetAt(i)->GetOwnerNode();
+    if (nsContentUtils::PositionIsBefore(aLinkingContent, sheetOwningNode)) {
       mProtoBinding->InsertStyleSheetAt(i, aSheet);
       break;
     }
@@ -161,7 +163,7 @@ ShadowRoot::InsertSheet(CSSStyleSheet* aSheet,
 }
 
 void
-ShadowRoot::RemoveSheet(CSSStyleSheet* aSheet)
+ShadowRoot::RemoveSheet(StyleSheetHandle aSheet)
 {
   mProtoBinding->RemoveStyleSheet(aSheet);
 
@@ -752,7 +754,14 @@ ShadowRootStyleSheetList::IndexedGetter(uint32_t aIndex, bool& aFound)
     return nullptr;
   }
 
-  return mShadowRoot->mProtoBinding->StyleSheetAt(aIndex);
+  // XXXheycam Return null until ServoStyleSheet implements the right
+  // DOM interfaces.
+  StyleSheetHandle sheet = mShadowRoot->mProtoBinding->StyleSheetAt(aIndex);
+  if (sheet->IsServo()) {
+    NS_ERROR("stylo: can't return ServoStyleSheets to script yet");
+    return nullptr;
+  }
+  return sheet->AsGecko();
 }
 
 uint32_t
diff --git a/dom/base/ShadowRoot.h b/dom/base/ShadowRoot.h
index a2b71244c5fd6..cbf4cfb088009 100644
--- a/dom/base/ShadowRoot.h
+++ b/dom/base/ShadowRoot.h
@@ -9,6 +9,7 @@
 
 #include "mozilla/dom/DocumentFragment.h"
 #include "mozilla/dom/StyleSheetList.h"
+#include "mozilla/StyleSheetHandle.h"
 #include "nsCOMPtr.h"
 #include "nsCycleCollectionParticipant.h"
 #include "nsTHashtable.h"
@@ -45,8 +46,8 @@ class ShadowRoot final : public DocumentFragment,
 
   void AddToIdTable(Element* aElement, nsIAtom* aId);
   void RemoveFromIdTable(Element* aElement, nsIAtom* aId);
-  void InsertSheet(CSSStyleSheet* aSheet, nsIContent* aLinkingContent);
-  void RemoveSheet(CSSStyleSheet* aSheet);
+  void InsertSheet(StyleSheetHandle aSheet, nsIContent* aLinkingContent);
+  void RemoveSheet(StyleSheetHandle aSheet);
   bool ApplyAuthorStyles();
   void SetApplyAuthorStyles(bool aApplyAuthorStyles);
   StyleSheetList* StyleSheets();
diff --git a/dom/base/nsContentSink.cpp b/dom/base/nsContentSink.cpp
index 1e40b73a37333..7a80464dfe884 100644
--- a/dom/base/nsContentSink.cpp
+++ b/dom/base/nsContentSink.cpp
@@ -213,7 +213,7 @@ nsContentSink::Init(nsIDocument* aDoc,
 }
 
 NS_IMETHODIMP
-nsContentSink::StyleSheetLoaded(CSSStyleSheet* aSheet,
+nsContentSink::StyleSheetLoaded(StyleSheetHandle aSheet,
                                 bool aWasAlternate,
                                 nsresult aStatus)
 {
diff --git a/dom/base/nsContentSink.h b/dom/base/nsContentSink.h
index e65c6f7edabfd..a5b1e2ab7c864 100644
--- a/dom/base/nsContentSink.h
+++ b/dom/base/nsContentSink.h
@@ -85,7 +85,7 @@ class nsContentSink : public nsICSSLoaderObserver,
   NS_DECL_NSITIMERCALLBACK
 
   // nsICSSLoaderObserver
-  NS_IMETHOD StyleSheetLoaded(mozilla::CSSStyleSheet* aSheet,
+  NS_IMETHOD StyleSheetLoaded(mozilla::StyleSheetHandle aSheet,
                               bool aWasAlternate,
                               nsresult aStatus) override;
 
diff --git a/dom/base/nsDocument.cpp b/dom/base/nsDocument.cpp
index 7032fbb875d12..fa9416bb5a29f 100644
--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -245,6 +245,8 @@
 #include "nsISupportsPrimitives.h"
 #include "mozilla/StyleSetHandle.h"
 #include "mozilla/StyleSetHandleInlines.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 
 #include "mozilla/DocLoadingTimelineMarker.h"
 
@@ -733,10 +735,16 @@ nsDOMStyleSheetList::IndexedGetter(uint32_t aIndex, bool& aFound)
   }
 
   aFound = true;
-  CSSStyleSheet* sheet = mDocument->GetStyleSheetAt(aIndex);
+  StyleSheetHandle sheet = mDocument->GetStyleSheetAt(aIndex);
   NS_ASSERTION(sheet, "Must have a sheet");
 
-  return static_cast<CSSStyleSheet*>(sheet);
+  // XXXheycam Return null until ServoStyleSheet implements the right DOM
+  // interfaces.
+  if (sheet->IsServo()) {
+    NS_ERROR("stylo: can't return a ServoStyleSheet to the DOM yet");
+    return nullptr;
+  }
+  return sheet->AsGecko();
 }
 
 void
@@ -746,7 +754,7 @@ nsDOMStyleSheetList::NodeWillBeDestroyed(const nsINode *aNode)
 }
 
 void
-nsDOMStyleSheetList::StyleSheetAdded(CSSStyleSheet* aStyleSheet,
+nsDOMStyleSheetList::StyleSheetAdded(StyleSheetHandle aStyleSheet,
                                      bool aDocumentSheet)
 {
   if (aDocumentSheet && -1 != mLength) {
@@ -755,7 +763,7 @@ nsDOMStyleSheetList::StyleSheetAdded(CSSStyleSheet* aStyleSheet,
 }
 
 void
-nsDOMStyleSheetList::StyleSheetRemoved(CSSStyleSheet* aStyleSheet,
+nsDOMStyleSheetList::StyleSheetRemoved(StyleSheetHandle aStyleSheet,
                                        bool aDocumentSheet)
 {
   if (aDocumentSheet && -1 != mLength) {
@@ -1322,9 +1330,14 @@ nsDOMStyleSheetSetList::EnsureFresh()
   int32_t count = mDocument->GetNumberOfStyleSheets();
   nsAutoString title;
   for (int32_t index = 0; index < count; index++) {
-    CSSStyleSheet* sheet = mDocument->GetStyleSheetAt(index);
+    StyleSheetHandle sheet = mDocument->GetStyleSheetAt(index);
     NS_ASSERTION(sheet, "Null sheet in sheet list!");
-    sheet->GetTitle(title);
+    // XXXheycam ServoStyleSheets don't expose their title yet.
+    if (sheet->IsServo()) {
+      NS_ERROR("stylo: ServoStyleSets don't expose their title yet");
+      continue;
+    }
+    sheet->AsGecko()->GetTitle(title);
     if (!title.IsEmpty() && !mNames.Contains(title) && !Add(title)) {
       return;
     }
@@ -1604,7 +1617,7 @@ nsDocument::~nsDocument()
   mCachedRootElement = nullptr;
 
   // Let the stylesheets know we're going away
-  for (CSSStyleSheet* sheet : mStyleSheets) {
+  for (StyleSheetHandle sheet : mStyleSheets) {
     sheet->SetOwningDocument(nullptr);
   }
   if (mAttrStyleSheet) {
@@ -2282,7 +2295,7 @@ void
 nsDocument::RemoveDocStyleSheetsFromStyleSets()
 {
   // The stylesheets should forget us
-  for (CSSStyleSheet* sheet : Reversed(mStyleSheets)) {
+  for (StyleSheetHandle sheet : Reversed(mStyleSheets)) {
     sheet->SetOwningDocument(nullptr);
 
     if (sheet->IsApplicable()) {
@@ -2297,11 +2310,11 @@ nsDocument::RemoveDocStyleSheetsFromStyleSets()
 
 void
 nsDocument::RemoveStyleSheetsFromStyleSets(
-    nsTArray<RefPtr<CSSStyleSheet>>& aSheets,
+    nsTArray<StyleSheetHandle::RefPtr>& aSheets,
     SheetType aType)
 {
   // The stylesheets should forget us
-  for (CSSStyleSheet* sheet : Reversed(aSheets)) {
+  for (StyleSheetHandle sheet : Reversed(aSheets)) {
     sheet->SetOwningDocument(nullptr);
 
     if (sheet->IsApplicable()) {
@@ -2368,10 +2381,10 @@ nsDocument::ResetStylesheetsToURI(nsIURI* aURI)
 
 static void
 AppendSheetsToStyleSet(StyleSetHandle aStyleSet,
-                       const nsTArray<RefPtr<CSSStyleSheet>>& aSheets,
+                       const nsTArray<StyleSheetHandle::RefPtr>& aSheets,
                        SheetType aType)
 {
-  for (CSSStyleSheet* sheet : Reversed(aSheets)) {
+  for (StyleSheetHandle sheet : Reversed(aSheets)) {
     aStyleSet->AppendStyleSheet(aType, sheet);
   }
 }
@@ -2384,7 +2397,7 @@ nsDocument::FillStyleSet(StyleSetHandle aStyleSet)
   NS_PRECONDITION(aStyleSet->SheetCount(SheetType::Doc) == 0,
                   "Style set already has document sheets?");
 
-  for (CSSStyleSheet* sheet : Reversed(mStyleSheets)) {
+  for (StyleSheetHandle sheet : Reversed(mStyleSheets)) {
     if (sheet->IsApplicable()) {
       aStyleSet->AddDocStyleSheet(sheet, this);
     }
@@ -2392,13 +2405,13 @@ nsDocument::FillStyleSet(StyleSetHandle aStyleSet)
 
   nsStyleSheetService *sheetService = nsStyleSheetService::GetInstance();
   if (sheetService) {
-    for (CSSStyleSheet* sheet : *sheetService->AuthorStyleSheets()) {
+    for (StyleSheetHandle sheet : *sheetService->AuthorStyleSheets()) {
       aStyleSet->AppendStyleSheet(SheetType::Doc, sheet);
     }
   }
 
   // Iterate backwards to maintain order
-  for (CSSStyleSheet* sheet : Reversed(mOnDemandBuiltInUASheets)) {
+  for (StyleSheetHandle sheet : Reversed(mOnDemandBuiltInUASheets)) {
     if (sheet->IsApplicable()) {
       aStyleSet->PrependStyleSheet(SheetType::Agent, sheet);
     }
@@ -4051,7 +4064,7 @@ nsDocument::RemoveChildAt(uint32_t aIndex, bool aNotify)
 }
 
 void
-nsDocument::EnsureOnDemandBuiltInUASheet(CSSStyleSheet* aSheet)
+nsDocument::EnsureOnDemandBuiltInUASheet(StyleSheetHandle aSheet)
 {
   if (mOnDemandBuiltInUASheets.Contains(aSheet)) {
     return;
@@ -4062,7 +4075,7 @@ nsDocument::EnsureOnDemandBuiltInUASheet(CSSStyleSheet* aSheet)
 }
 
 void
-nsDocument::AddOnDemandBuiltInUASheet(CSSStyleSheet* aSheet)
+nsDocument::AddOnDemandBuiltInUASheet(StyleSheetHandle aSheet)
 {
   MOZ_ASSERT(!mOnDemandBuiltInUASheets.Contains(aSheet));
 
@@ -4091,20 +4104,20 @@ nsDocument::GetNumberOfStyleSheets() const
   return mStyleSheets.Length();
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsDocument::GetStyleSheetAt(int32_t aIndex) const
 {
-  return mStyleSheets.SafeElementAt(aIndex, nullptr);
+  return mStyleSheets.SafeElementAt(aIndex, StyleSheetHandle());
 }
 
 int32_t
-nsDocument::GetIndexOfStyleSheet(CSSStyleSheet* aSheet) const
+nsDocument::GetIndexOfStyleSheet(StyleSheetHandle aSheet) const
 {
   return mStyleSheets.IndexOf(aSheet);
 }
 
 void
-nsDocument::AddStyleSheetToStyleSets(CSSStyleSheet* aSheet)
+nsDocument::AddStyleSheetToStyleSets(StyleSheetHandle aSheet)
 {
   nsCOMPtr<nsIPresShell> shell = GetShell();
   if (shell) {
@@ -4117,7 +4130,11 @@ nsDocument::AddStyleSheetToStyleSets(CSSStyleSheet* aSheet)
     className##Init init;                                                     \
     init.mBubbles = true;                                                     \
     init.mCancelable = true;                                                  \
-    init.mStylesheet = aSheet;                                                \
+    /* XXXheycam ServoStyleSheet doesn't implement DOM interfaces yet */      \
+    if (aSheet->IsServo()) {                                                  \
+      NS_ERROR("stylo: can't dispatch events for ServoStyleSheets yet");      \
+    }                                                                         \
+    init.mStylesheet = aSheet->IsGecko() ? aSheet->AsGecko() : nullptr;       \
     init.memberName = argName;                                                \
                                                                               \
     RefPtr<className> event =                                               \
@@ -4131,7 +4148,7 @@ nsDocument::AddStyleSheetToStyleSets(CSSStyleSheet* aSheet)
   } while (0);
 
 void
-nsDocument::NotifyStyleSheetAdded(CSSStyleSheet* aSheet, bool aDocumentSheet)
+nsDocument::NotifyStyleSheetAdded(StyleSheetHandle aSheet, bool aDocumentSheet)
 {
   NS_DOCUMENT_NOTIFY_OBSERVERS(StyleSheetAdded, (aSheet, aDocumentSheet));
 
@@ -4144,7 +4161,7 @@ nsDocument::NotifyStyleSheetAdded(CSSStyleSheet* aSheet, bool aDocumentSheet)
 }
 
 void
-nsDocument::NotifyStyleSheetRemoved(CSSStyleSheet* aSheet, bool aDocumentSheet)
+nsDocument::NotifyStyleSheetRemoved(StyleSheetHandle aSheet, bool aDocumentSheet)
 {
   NS_DOCUMENT_NOTIFY_OBSERVERS(StyleSheetRemoved, (aSheet, aDocumentSheet));
 
@@ -4157,7 +4174,7 @@ nsDocument::NotifyStyleSheetRemoved(CSSStyleSheet* aSheet, bool aDocumentSheet)
 }
 
 void
-nsDocument::AddStyleSheet(CSSStyleSheet* aSheet)
+nsDocument::AddStyleSheet(StyleSheetHandle aSheet)
 {
   NS_PRECONDITION(aSheet, "null arg");
   mStyleSheets.AppendElement(aSheet);
@@ -4171,7 +4188,7 @@ nsDocument::AddStyleSheet(CSSStyleSheet* aSheet)
 }
 
 void
-nsDocument::RemoveStyleSheetFromStyleSets(CSSStyleSheet* aSheet)
+nsDocument::RemoveStyleSheetFromStyleSets(StyleSheetHandle aSheet)
 {
   nsCOMPtr<nsIPresShell> shell = GetShell();
   if (shell) {
@@ -4180,10 +4197,10 @@ nsDocument::RemoveStyleSheetFromStyleSets(CSSStyleSheet* aSheet)
 }
 
 void
-nsDocument::RemoveStyleSheet(CSSStyleSheet* aSheet)
+nsDocument::RemoveStyleSheet(StyleSheetHandle aSheet)
 {
   NS_PRECONDITION(aSheet, "null arg");
-  RefPtr<CSSStyleSheet> sheet = aSheet; // hold ref so it won't die too soon
+  StyleSheetHandle::RefPtr sheet = aSheet; // hold ref so it won't die too soon
 
   if (!mStyleSheets.RemoveElement(aSheet)) {
     NS_ASSERTION(mInUnlinkOrDeletion, "stylesheet not found");
@@ -4202,8 +4219,8 @@ nsDocument::RemoveStyleSheet(CSSStyleSheet* aSheet)
 }
 
 void
-nsDocument::UpdateStyleSheets(nsTArray<RefPtr<CSSStyleSheet>>& aOldSheets,
-                              nsTArray<RefPtr<CSSStyleSheet>>& aNewSheets)
+nsDocument::UpdateStyleSheets(nsTArray<StyleSheetHandle::RefPtr>& aOldSheets,
+                              nsTArray<StyleSheetHandle::RefPtr>& aNewSheets)
 {
   BeginUpdate(UPDATE_STYLE);
 
@@ -4212,7 +4229,7 @@ nsDocument::UpdateStyleSheets(nsTArray<RefPtr<CSSStyleSheet>>& aOldSheets,
                   "The lists must be the same length!");
   int32_t count = aOldSheets.Length();
 
-  RefPtr<CSSStyleSheet> oldSheet;
+  StyleSheetHandle::RefPtr oldSheet;
   int32_t i;
   for (i = 0; i < count; ++i) {
     oldSheet = aOldSheets[i];
@@ -4223,7 +4240,7 @@ nsDocument::UpdateStyleSheets(nsTArray<RefPtr<CSSStyleSheet>>& aOldSheets,
     RemoveStyleSheet(oldSheet);  // This does the right notifications
 
     // Now put the new one in its place.  If it's null, just ignore it.
-    CSSStyleSheet* newSheet = aNewSheets[i];
+    StyleSheetHandle newSheet = aNewSheets[i];
     if (newSheet) {
       mStyleSheets.InsertElementAt(oldIndex, newSheet);
       newSheet->SetOwningDocument(this);
@@ -4239,7 +4256,7 @@ nsDocument::UpdateStyleSheets(nsTArray<RefPtr<CSSStyleSheet>>& aOldSheets,
 }
 
 void
-nsDocument::InsertStyleSheetAt(CSSStyleSheet* aSheet, int32_t aIndex)
+nsDocument::InsertStyleSheetAt(StyleSheetHandle aSheet, int32_t aIndex)
 {
   NS_PRECONDITION(aSheet, "null ptr");
 
@@ -4256,7 +4273,7 @@ nsDocument::InsertStyleSheetAt(CSSStyleSheet* aSheet, int32_t aIndex)
 
 
 void
-nsDocument::SetStyleSheetApplicableState(CSSStyleSheet* aSheet,
+nsDocument::SetStyleSheetApplicableState(StyleSheetHandle aSheet,
                                          bool aApplicable)
 {
   NS_PRECONDITION(aSheet, "null arg");
@@ -4322,7 +4339,7 @@ ConvertAdditionalSheetType(nsIDocument::additionalSheetType aType)
 }
 
 static int32_t
-FindSheet(const nsTArray<RefPtr<CSSStyleSheet>>& aSheets, nsIURI* aSheetURI)
+FindSheet(const nsTArray<StyleSheetHandle::RefPtr>& aSheets, nsIURI* aSheetURI)
 {
   for (int32_t i = aSheets.Length() - 1; i >= 0; i-- ) {
     bool bEqual;
@@ -4366,9 +4383,8 @@ nsDocument::LoadAdditionalStyleSheet(additionalSheetType aType,
       MOZ_CRASH("impossible value for aType");
   }
 
-  RefPtr<CSSStyleSheet> sheet;
-  nsresult rv = loader->LoadSheetSync(aSheetURI, parsingMode, true,
-                                      getter_AddRefs(sheet));
+  StyleSheetHandle::RefPtr sheet;
+  nsresult rv = loader->LoadSheetSync(aSheetURI, parsingMode, true, &sheet);
   NS_ENSURE_SUCCESS(rv, rv);
 
   sheet->SetOwningDocument(this);
@@ -4378,7 +4394,7 @@ nsDocument::LoadAdditionalStyleSheet(additionalSheetType aType,
 }
 
 nsresult
-nsDocument::AddAdditionalStyleSheet(additionalSheetType aType, CSSStyleSheet* aSheet)
+nsDocument::AddAdditionalStyleSheet(additionalSheetType aType, StyleSheetHandle aSheet)
 {
   if (mAdditionalSheets[aType].Contains(aSheet))
     return NS_ERROR_INVALID_ARG;
@@ -4407,11 +4423,11 @@ nsDocument::RemoveAdditionalStyleSheet(additionalSheetType aType, nsIURI* aSheet
 {
   MOZ_ASSERT(aSheetURI);
 
-  nsTArray<RefPtr<CSSStyleSheet>>& sheets = mAdditionalSheets[aType];
+  nsTArray<StyleSheetHandle::RefPtr>& sheets = mAdditionalSheets[aType];
 
   int32_t i = FindSheet(mAdditionalSheets[aType], aSheetURI);
   if (i >= 0) {
-    RefPtr<CSSStyleSheet> sheetRef = sheets[i];
+    StyleSheetHandle::RefPtr sheetRef = sheets[i];
     sheets.RemoveElementAt(i);
 
     BeginUpdate(UPDATE_STYLE);
@@ -4433,10 +4449,10 @@ nsDocument::RemoveAdditionalStyleSheet(additionalSheetType aType, nsIURI* aSheet
   }
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsDocument::FirstAdditionalAuthorSheet()
 {
-  return mAdditionalSheets[eAuthorSheet].SafeElementAt(0, nullptr);
+  return mAdditionalSheets[eAuthorSheet].SafeElementAt(0, StyleSheetHandle());
 }
 
 nsIGlobalObject*
@@ -5192,7 +5208,7 @@ nsDocument::DocumentStatesChanged(EventStates aStateMask)
 }
 
 void
-nsDocument::StyleRuleChanged(CSSStyleSheet* aSheet,
+nsDocument::StyleRuleChanged(StyleSheetHandle aSheet,
                              css::Rule* aStyleRule)
 {
   NS_DOCUMENT_NOTIFY_OBSERVERS(StyleRuleChanged, (aSheet));
@@ -5206,7 +5222,7 @@ nsDocument::StyleRuleChanged(CSSStyleSheet* aSheet,
 }
 
 void
-nsDocument::StyleRuleAdded(CSSStyleSheet* aSheet,
+nsDocument::StyleRuleAdded(StyleSheetHandle aSheet,
                            css::Rule* aStyleRule)
 {
   NS_DOCUMENT_NOTIFY_OBSERVERS(StyleRuleAdded, (aSheet));
@@ -5221,7 +5237,7 @@ nsDocument::StyleRuleAdded(CSSStyleSheet* aSheet,
 }
 
 void
-nsDocument::StyleRuleRemoved(CSSStyleSheet* aSheet,
+nsDocument::StyleRuleRemoved(StyleSheetHandle aSheet,
                              css::Rule* aStyleRule)
 {
   NS_DOCUMENT_NOTIFY_OBSERVERS(StyleRuleRemoved, (aSheet));
@@ -6462,17 +6478,23 @@ nsIDocument::GetSelectedStyleSheetSet(nsAString& aSheetSet)
   int32_t count = GetNumberOfStyleSheets();
   nsAutoString title;
   for (int32_t index = 0; index < count; index++) {
-    CSSStyleSheet* sheet = GetStyleSheetAt(index);
+    StyleSheetHandle sheet = GetStyleSheetAt(index);
     NS_ASSERTION(sheet, "Null sheet in sheet list!");
 
+    // XXXheycam Make this work with ServoStyleSheets.
+    if (sheet->IsServo()) {
+      NS_ERROR("stylo: can't handle alternate ServoStyleSheets yet");
+      continue;
+    }
+
     bool disabled;
-    sheet->GetDisabled(&disabled);
+    sheet->AsGecko()->GetDisabled(&disabled);
     if (disabled) {
       // Disabled sheets don't affect the currently selected set
       continue;
     }
 
-    sheet->GetTitle(title);
+    sheet->AsGecko()->GetTitle(title);
 
     if (aSheetSet.IsEmpty()) {
       aSheetSet = title;
@@ -6576,11 +6598,18 @@ nsDocument::EnableStyleSheetsForSetInternal(const nsAString& aSheetSet,
   int32_t count = GetNumberOfStyleSheets();
   nsAutoString title;
   for (int32_t index = 0; index < count; index++) {
-    CSSStyleSheet* sheet = GetStyleSheetAt(index);
+    StyleSheetHandle sheet = GetStyleSheetAt(index);
     NS_ASSERTION(sheet, "Null sheet in sheet list!");
-    sheet->GetTitle(title);
+
+    // XXXheycam Make this work with ServoStyleSheets.
+    if (sheet->IsServo()) {
+      NS_ERROR("stylo: can't handle alternate ServoStyleSheets yet");
+      continue;
+    }
+
+    sheet->AsGecko()->GetTitle(title);
     if (!title.IsEmpty()) {
-      sheet->SetEnabled(title.Equals(aSheetSet));
+      sheet->AsGecko()->SetEnabled(title.Equals(aSheetSet));
     }
   }
   if (aUpdateCSSLoader) {
@@ -9854,7 +9883,7 @@ class StubCSSLoaderObserver final : public nsICSSLoaderObserver {
   ~StubCSSLoaderObserver() {}
 public:
   NS_IMETHOD
-  StyleSheetLoaded(CSSStyleSheet*, bool, nsresult) override
+  StyleSheetLoaded(StyleSheetHandle, bool, nsresult) override
   {
     return NS_OK;
   }
@@ -9883,12 +9912,12 @@ nsDocument::PreloadStyle(nsIURI* uri, const nsAString& charset,
 
 nsresult
 nsDocument::LoadChromeSheetSync(nsIURI* uri, bool isAgentSheet,
-                                CSSStyleSheet** sheet)
+                                mozilla::StyleSheetHandle::RefPtr* aSheet)
 {
   css::SheetParsingMode mode =
     isAgentSheet ? css::eAgentSheetFeatures
                  : css::eAuthorSheetFeatures;
-  return CSSLoader()->LoadSheetSync(uri, mode, isAgentSheet, sheet);
+  return CSSLoader()->LoadSheetSync(uri, mode, isAgentSheet, aSheet);
 }
 
 class nsDelayedEventDispatcher : public nsRunnable
@@ -10214,28 +10243,38 @@ nsIDocument::CreateStaticClone(nsIDocShell* aCloneContainer)
 
       int32_t sheetsCount = GetNumberOfStyleSheets();
       for (int32_t i = 0; i < sheetsCount; ++i) {
-        RefPtr<CSSStyleSheet> sheet = GetStyleSheetAt(i);
+        StyleSheetHandle::RefPtr sheet = GetStyleSheetAt(i);
         if (sheet) {
           if (sheet->IsApplicable()) {
-            RefPtr<CSSStyleSheet> clonedSheet =
-              sheet->Clone(nullptr, nullptr, clonedDoc, nullptr);
-            NS_WARN_IF_FALSE(clonedSheet, "Cloning a stylesheet didn't work!");
-            if (clonedSheet) {
-              clonedDoc->AddStyleSheet(clonedSheet);
+            // XXXheycam Need to make ServoStyleSheet cloning work.
+            if (sheet->IsGecko()) {
+              RefPtr<CSSStyleSheet> clonedSheet =
+                sheet->AsGecko()->Clone(nullptr, nullptr, clonedDoc, nullptr);
+              NS_WARN_IF_FALSE(clonedSheet, "Cloning a stylesheet didn't work!");
+              if (clonedSheet) {
+                clonedDoc->AddStyleSheet(clonedSheet);
+              }
+            } else {
+              NS_ERROR("stylo: ServoStyleSheet doesn't support cloning");
             }
           }
         }
       }
 
       // Iterate backwards to maintain order
-      for (CSSStyleSheet* sheet : Reversed(thisAsDoc->mOnDemandBuiltInUASheets)) {
+      for (StyleSheetHandle sheet : Reversed(thisAsDoc->mOnDemandBuiltInUASheets)) {
         if (sheet) {
           if (sheet->IsApplicable()) {
-            RefPtr<CSSStyleSheet> clonedSheet =
-              sheet->Clone(nullptr, nullptr, clonedDoc, nullptr);
-            NS_WARN_IF_FALSE(clonedSheet, "Cloning a stylesheet didn't work!");
-            if (clonedSheet) {
-              clonedDoc->AddOnDemandBuiltInUASheet(clonedSheet);
+            // XXXheycam Need to make ServoStyleSheet cloning work.
+            if (sheet->IsGecko()) {
+              RefPtr<CSSStyleSheet> clonedSheet =
+                sheet->AsGecko()->Clone(nullptr, nullptr, clonedDoc, nullptr);
+              NS_WARN_IF_FALSE(clonedSheet, "Cloning a stylesheet didn't work!");
+              if (clonedSheet) {
+                clonedDoc->AddOnDemandBuiltInUASheet(clonedSheet);
+              }
+            } else {
+              NS_ERROR("stylo: ServoStyleSheet doesn't support cloning");
             }
           }
         }
@@ -12366,7 +12405,7 @@ nsDocument::OnAppThemeChanged()
   }
 
   for (int32_t i = 0; i < GetNumberOfStyleSheets(); i++) {
-    RefPtr<CSSStyleSheet> sheet = GetStyleSheetAt(i);
+    StyleSheetHandle::RefPtr sheet = GetStyleSheetAt(i);
     if (!sheet) {
       continue;
     }
@@ -12733,12 +12772,12 @@ nsIDocument::DocAddSizeOfIncludingThis(nsWindowSizes* aWindowSizes) const
 }
 
 static size_t
-SizeOfOwnedSheetArrayExcludingThis(const nsTArray<RefPtr<CSSStyleSheet>>& aSheets,
+SizeOfOwnedSheetArrayExcludingThis(const nsTArray<StyleSheetHandle::RefPtr>& aSheets,
                                    MallocSizeOf aMallocSizeOf)
 {
   size_t n = 0;
   n += aSheets.ShallowSizeOfExcludingThis(aMallocSizeOf);
-  for (CSSStyleSheet* sheet : aSheets) {
+  for (StyleSheetHandle sheet : aSheets) {
     if (!sheet->GetOwningDocument()) {
       // Avoid over-reporting shared sheets.
       continue;
@@ -13419,3 +13458,15 @@ nsIDocument::ReportHasScrollLinkedEffect()
                                   this, nsContentUtils::eLAYOUT_PROPERTIES,
                                   "ScrollLinkedEffectFound2");
 }
+
+mozilla::StyleBackendType
+nsIDocument::GetStyleBackendType() const
+{
+  if (!mPresShell) {
+#ifdef MOZ_STYLO
+    NS_WARNING("GetStyleBackendType() called on document without a pres shell");
+#endif
+    return StyleBackendType::Gecko;
+  }
+  return mPresShell->StyleSet()->BackendType();
+}
diff --git a/dom/base/nsDocument.h b/dom/base/nsDocument.h
index 328dc5e835754..b3dc7f0f7ece9 100644
--- a/dom/base/nsDocument.h
+++ b/dom/base/nsDocument.h
@@ -797,36 +797,36 @@ class nsDocument : public nsIDocument,
   virtual Element* FindContentForSubDocument(nsIDocument *aDocument) const override;
   virtual Element* GetRootElementInternal() const override;
 
-  virtual void EnsureOnDemandBuiltInUASheet(mozilla::CSSStyleSheet* aSheet) override;
+  virtual void EnsureOnDemandBuiltInUASheet(mozilla::StyleSheetHandle aSheet) override;
 
   /**
    * Get the (document) style sheets owned by this document.
    * These are ordered, highest priority last
    */
   virtual int32_t GetNumberOfStyleSheets() const override;
-  virtual mozilla::CSSStyleSheet* GetStyleSheetAt(int32_t aIndex) const override;
-  virtual int32_t GetIndexOfStyleSheet(mozilla::CSSStyleSheet* aSheet) const override;
-  virtual void AddStyleSheet(mozilla::CSSStyleSheet* aSheet) override;
-  virtual void RemoveStyleSheet(mozilla::CSSStyleSheet* aSheet) override;
+  virtual mozilla::StyleSheetHandle GetStyleSheetAt(int32_t aIndex) const override;
+  virtual int32_t GetIndexOfStyleSheet(mozilla::StyleSheetHandle aSheet) const override;
+  virtual void AddStyleSheet(mozilla::StyleSheetHandle aSheet) override;
+  virtual void RemoveStyleSheet(mozilla::StyleSheetHandle aSheet) override;
 
   virtual void UpdateStyleSheets(
-      nsTArray<RefPtr<mozilla::CSSStyleSheet>>& aOldSheets,
-      nsTArray<RefPtr<mozilla::CSSStyleSheet>>& aNewSheets) override;
-  virtual void AddStyleSheetToStyleSets(mozilla::CSSStyleSheet* aSheet);
-  virtual void RemoveStyleSheetFromStyleSets(mozilla::CSSStyleSheet* aSheet);
+      nsTArray<mozilla::StyleSheetHandle::RefPtr>& aOldSheets,
+      nsTArray<mozilla::StyleSheetHandle::RefPtr>& aNewSheets) override;
+  virtual void AddStyleSheetToStyleSets(mozilla::StyleSheetHandle aSheet);
+  virtual void RemoveStyleSheetFromStyleSets(mozilla::StyleSheetHandle aSheet);
 
-  virtual void InsertStyleSheetAt(mozilla::CSSStyleSheet* aSheet,
+  virtual void InsertStyleSheetAt(mozilla::StyleSheetHandle aSheet,
                                   int32_t aIndex) override;
-  virtual void SetStyleSheetApplicableState(mozilla::CSSStyleSheet* aSheet,
+  virtual void SetStyleSheetApplicableState(mozilla::StyleSheetHandle aSheet,
                                             bool aApplicable) override;
 
   virtual nsresult LoadAdditionalStyleSheet(additionalSheetType aType,
                                             nsIURI* aSheetURI) override;
   virtual nsresult AddAdditionalStyleSheet(additionalSheetType aType,
-                                           mozilla::CSSStyleSheet* aSheet) override;
+                                           mozilla::StyleSheetHandle aSheet) override;
   virtual void RemoveAdditionalStyleSheet(additionalSheetType aType,
                                           nsIURI* sheetURI) override;
-  virtual mozilla::CSSStyleSheet* FirstAdditionalAuthorSheet() override;
+  virtual mozilla::StyleSheetHandle FirstAdditionalAuthorSheet() override;
 
   virtual nsIChannel* GetChannel() const override {
     return mChannel;
@@ -886,11 +886,11 @@ class nsDocument : public nsIDocument,
   virtual void DocumentStatesChanged(
                  mozilla::EventStates aStateMask) override;
 
-  virtual void StyleRuleChanged(mozilla::CSSStyleSheet* aStyleSheet,
+  virtual void StyleRuleChanged(mozilla::StyleSheetHandle aStyleSheet,
                                 mozilla::css::Rule* aStyleRule) override;
-  virtual void StyleRuleAdded(mozilla::CSSStyleSheet* aStyleSheet,
+  virtual void StyleRuleAdded(mozilla::StyleSheetHandle aStyleSheet,
                               mozilla::css::Rule* aStyleRule) override;
-  virtual void StyleRuleRemoved(mozilla::CSSStyleSheet* aStyleSheet,
+  virtual void StyleRuleRemoved(mozilla::StyleSheetHandle aStyleSheet,
                                 mozilla::css::Rule* aStyleRule) override;
 
   virtual void FlushPendingNotifications(mozFlushType aType) override;
@@ -962,7 +962,7 @@ class nsDocument : public nsIDocument,
   void ReportUseCounters();
 
 private:
-  void AddOnDemandBuiltInUASheet(mozilla::CSSStyleSheet* aSheet);
+  void AddOnDemandBuiltInUASheet(mozilla::StyleSheetHandle aSheet);
   nsRadioGroupStruct* GetRadioGroupInternal(const nsAString& aName) const;
   void SendToConsole(nsCOMArray<nsISecurityConsoleMessage>& aMessages);
 
@@ -1144,7 +1144,7 @@ class nsDocument : public nsIDocument,
                             const nsAString& aIntegrity) override;
 
   virtual nsresult LoadChromeSheetSync(nsIURI* uri, bool isAgentSheet,
-                                       mozilla::CSSStyleSheet** sheet) override;
+                                       mozilla::StyleSheetHandle::RefPtr* aSheet) override;
 
   virtual nsISupports* GetCurrentContentSink() override;
 
@@ -1494,7 +1494,7 @@ class nsDocument : public nsIDocument,
 
   void RemoveDocStyleSheetsFromStyleSets();
   void RemoveStyleSheetsFromStyleSets(
-      nsTArray<RefPtr<mozilla::CSSStyleSheet>>& aSheets,
+      nsTArray<mozilla::StyleSheetHandle::RefPtr>& aSheets,
       mozilla::SheetType aType);
   void ResetStylesheetsToURI(nsIURI* aURI);
   void FillStyleSet(mozilla::StyleSetHandle aStyleSet);
@@ -1548,9 +1548,9 @@ class nsDocument : public nsIDocument,
   // EndLoad() has already happened.
   nsWeakPtr mWeakSink;
 
-  nsTArray<RefPtr<mozilla::CSSStyleSheet>> mStyleSheets;
-  nsTArray<RefPtr<mozilla::CSSStyleSheet>> mOnDemandBuiltInUASheets;
-  nsTArray<RefPtr<mozilla::CSSStyleSheet>> mAdditionalSheets[AdditionalSheetTypeCount];
+  nsTArray<mozilla::StyleSheetHandle::RefPtr> mStyleSheets;
+  nsTArray<mozilla::StyleSheetHandle::RefPtr> mOnDemandBuiltInUASheets;
+  nsTArray<mozilla::StyleSheetHandle::RefPtr> mAdditionalSheets[AdditionalSheetTypeCount];
 
   // Array of observers
   nsTObserverArray<nsIDocumentObserver*> mObservers;
@@ -1709,8 +1709,8 @@ class nsDocument : public nsIDocument,
   friend class nsUnblockOnloadEvent;
   // Recomputes the visibility state but doesn't set the new value.
   mozilla::dom::VisibilityState GetVisibilityState() const;
-  void NotifyStyleSheetAdded(mozilla::CSSStyleSheet* aSheet, bool aDocumentSheet);
-  void NotifyStyleSheetRemoved(mozilla::CSSStyleSheet* aSheet, bool aDocumentSheet);
+  void NotifyStyleSheetAdded(mozilla::StyleSheetHandle aSheet, bool aDocumentSheet);
+  void NotifyStyleSheetRemoved(mozilla::StyleSheetHandle aSheet, bool aDocumentSheet);
 
   void PostUnblockOnloadEvent();
   void DoUnblockOnload();
diff --git a/dom/base/nsIDocument.h b/dom/base/nsIDocument.h
index 797c3188a5af6..46cebf82c5ff1 100644
--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -34,6 +34,8 @@
 #include "prclist.h"
 #include "mozilla/UniquePtr.h"
 #include "mozilla/CORSMode.h"
+#include "mozilla/StyleBackendType.h"
+#include "mozilla/StyleSheetHandle.h"
 #include <bitset>                        // for member
 
 class gfxUserFontSet;
@@ -918,7 +920,7 @@ class nsIDocument : public nsINode
    * TODO We can get rid of the whole concept of delayed loading if we fix
    * bug 77999.
    */
-  virtual void EnsureOnDemandBuiltInUASheet(mozilla::CSSStyleSheet* aSheet) = 0;
+  virtual void EnsureOnDemandBuiltInUASheet(mozilla::StyleSheetHandle aSheet) = 0;
 
   /**
    * Get the number of (document) stylesheets
@@ -934,7 +936,7 @@ class nsIDocument : public nsINode
    * @return the stylesheet at aIndex.  Null if aIndex is out of range.
    * @throws no exceptions
    */
-  virtual mozilla::CSSStyleSheet* GetStyleSheetAt(int32_t aIndex) const = 0;
+  virtual mozilla::StyleSheetHandle GetStyleSheetAt(int32_t aIndex) const = 0;
 
   /**
    * Insert a sheet at a particular spot in the stylesheet list (zero-based)
@@ -943,7 +945,7 @@ class nsIDocument : public nsINode
    *   adjusted for the "special" sheets.
    * @throws no exceptions
    */
-  virtual void InsertStyleSheetAt(mozilla::CSSStyleSheet* aSheet,
+  virtual void InsertStyleSheetAt(mozilla::StyleSheetHandle aSheet,
                                   int32_t aIndex) = 0;
 
   /**
@@ -952,7 +954,7 @@ class nsIDocument : public nsINode
    * @param aSheet the sheet to get the index of
    * @return aIndex the index of the sheet in the full list
    */
-  virtual int32_t GetIndexOfStyleSheet(mozilla::CSSStyleSheet* aSheet) const = 0;
+  virtual int32_t GetIndexOfStyleSheet(mozilla::StyleSheetHandle aSheet) const = 0;
 
   /**
    * Replace the stylesheets in aOldSheets with the stylesheets in
@@ -963,24 +965,24 @@ class nsIDocument : public nsINode
    * will simply be removed.
    */
   virtual void UpdateStyleSheets(
-      nsTArray<RefPtr<mozilla::CSSStyleSheet>>& aOldSheets,
-      nsTArray<RefPtr<mozilla::CSSStyleSheet>>& aNewSheets) = 0;
+      nsTArray<mozilla::StyleSheetHandle::RefPtr>& aOldSheets,
+      nsTArray<mozilla::StyleSheetHandle::RefPtr>& aNewSheets) = 0;
 
   /**
    * Add a stylesheet to the document
    */
-  virtual void AddStyleSheet(mozilla::CSSStyleSheet* aSheet) = 0;
+  virtual void AddStyleSheet(mozilla::StyleSheetHandle aSheet) = 0;
 
   /**
    * Remove a stylesheet from the document
    */
-  virtual void RemoveStyleSheet(mozilla::CSSStyleSheet* aSheet) = 0;
+  virtual void RemoveStyleSheet(mozilla::StyleSheetHandle aSheet) = 0;
 
   /**
    * Notify the document that the applicable state of the sheet changed
    * and that observers should be notified and style sets updated
    */
-  virtual void SetStyleSheetApplicableState(mozilla::CSSStyleSheet* aSheet,
+  virtual void SetStyleSheetApplicableState(mozilla::StyleSheetHandle aSheet,
                                             bool aApplicable) = 0;
 
   enum additionalSheetType {
@@ -993,10 +995,10 @@ class nsIDocument : public nsINode
   virtual nsresult LoadAdditionalStyleSheet(additionalSheetType aType,
                                             nsIURI* aSheetURI) = 0;
   virtual nsresult AddAdditionalStyleSheet(additionalSheetType aType,
-                                           mozilla::CSSStyleSheet* aSheet) = 0;
+                                           mozilla::StyleSheetHandle aSheet) = 0;
   virtual void RemoveAdditionalStyleSheet(additionalSheetType aType,
                                           nsIURI* sheetURI) = 0;
-  virtual mozilla::CSSStyleSheet* FirstAdditionalAuthorSheet() = 0;
+  virtual mozilla::StyleSheetHandle FirstAdditionalAuthorSheet() = 0;
 
   /**
    * Get this document's CSSLoader.  This is guaranteed to not return null.
@@ -1005,6 +1007,8 @@ class nsIDocument : public nsINode
     return mCSSLoader;
   }
 
+  mozilla::StyleBackendType GetStyleBackendType() const;
+
   /**
    * Get this document's StyleImageLoader.  This is guaranteed to not return null.
    */
@@ -1292,11 +1296,11 @@ class nsIDocument : public nsINode
 
   // Observation hooks for style data to propagate notifications
   // to document observers
-  virtual void StyleRuleChanged(mozilla::CSSStyleSheet* aStyleSheet,
+  virtual void StyleRuleChanged(mozilla::StyleSheetHandle aStyleSheet,
                                 mozilla::css::Rule* aStyleRule) = 0;
-  virtual void StyleRuleAdded(mozilla::CSSStyleSheet* aStyleSheet,
+  virtual void StyleRuleAdded(mozilla::StyleSheetHandle aStyleSheet,
                               mozilla::css::Rule* aStyleRule) = 0;
-  virtual void StyleRuleRemoved(mozilla::CSSStyleSheet* aStyleSheet,
+  virtual void StyleRuleRemoved(mozilla::StyleSheetHandle aStyleSheet,
                                 mozilla::css::Rule* aStyleRule) = 0;
 
   /**
@@ -2091,7 +2095,7 @@ class nsIDocument : public nsINode
    * DO NOT USE FOR UNTRUSTED CONTENT.
    */
   virtual nsresult LoadChromeSheetSync(nsIURI* aURI, bool aIsAgentSheet,
-                                       mozilla::CSSStyleSheet** aSheet) = 0;
+                                       mozilla::StyleSheetHandle::RefPtr* aSheet) = 0;
 
   /**
    * Returns true if the locale used for the document specifies a direction of
diff --git a/dom/base/nsIDocumentObserver.h b/dom/base/nsIDocumentObserver.h
index 5d6d2542a25ee..0aed2a7cca037 100644
--- a/dom/base/nsIDocumentObserver.h
+++ b/dom/base/nsIDocumentObserver.h
@@ -7,6 +7,7 @@
 #define nsIDocumentObserver_h___
 
 #include "mozilla/EventStates.h"
+#include "mozilla/StyleSheetHandle.h"
 #include "nsISupports.h"
 #include "nsIMutationObserver.h"
 
@@ -14,7 +15,6 @@ class nsIContent;
 class nsIDocument;
 
 namespace mozilla {
-class CSSStyleSheet;
 namespace css {
 class Rule;
 } // namespace css
@@ -100,7 +100,7 @@ class nsIDocumentObserver : public nsIMutationObserver
    * @param aDocumentSheet True if sheet is in document's style sheet list,
    *                       false if sheet is not (i.e., UA or user sheet)
    */
-  virtual void StyleSheetAdded(mozilla::CSSStyleSheet* aStyleSheet,
+  virtual void StyleSheetAdded(mozilla::StyleSheetHandle aStyleSheet,
                                bool aDocumentSheet) = 0;
 
   /**
@@ -113,7 +113,7 @@ class nsIDocumentObserver : public nsIMutationObserver
    * @param aDocumentSheet True if sheet is in document's style sheet list,
    *                       false if sheet is not (i.e., UA or user sheet)
    */
-  virtual void StyleSheetRemoved(mozilla::CSSStyleSheet* aStyleSheet,
+  virtual void StyleSheetRemoved(mozilla::StyleSheetHandle aStyleSheet,
                                  bool aDocumentSheet) = 0;
   
   /**
@@ -125,7 +125,7 @@ class nsIDocumentObserver : public nsIMutationObserver
    *
    * @param aStyleSheet the StyleSheet that has changed state
    */
-  virtual void StyleSheetApplicableStateChanged(mozilla::CSSStyleSheet* aStyleSheet) = 0;
+  virtual void StyleSheetApplicableStateChanged(mozilla::StyleSheetHandle aStyleSheet) = 0;
 
   /**
    * A StyleRule has just been modified within a style sheet.
@@ -136,7 +136,7 @@ class nsIDocumentObserver : public nsIMutationObserver
    *
    * @param aStyleSheet the StyleSheet that contians the rule
    */
-  virtual void StyleRuleChanged(mozilla::CSSStyleSheet* aStyleSheet) = 0;
+  virtual void StyleRuleChanged(mozilla::StyleSheetHandle aStyleSheet) = 0;
 
   /**
    * A StyleRule has just been added to a style sheet.
@@ -147,7 +147,7 @@ class nsIDocumentObserver : public nsIMutationObserver
    *
    * @param aStyleSheet the StyleSheet that has been modified
    */
-  virtual void StyleRuleAdded(mozilla::CSSStyleSheet* aStyleSheet) = 0;
+  virtual void StyleRuleAdded(mozilla::StyleSheetHandle aStyleSheet) = 0;
 
   /**
    * A StyleRule has just been removed from a style sheet.
@@ -158,7 +158,7 @@ class nsIDocumentObserver : public nsIMutationObserver
    *
    * @param aStyleSheet the StyleSheet that has been modified
    */
-  virtual void StyleRuleRemoved(mozilla::CSSStyleSheet* aStyleSheet) = 0;
+  virtual void StyleRuleRemoved(mozilla::StyleSheetHandle aStyleSheet) = 0;
 };
 
 NS_DEFINE_STATIC_IID_ACCESSOR(nsIDocumentObserver, NS_IDOCUMENT_OBSERVER_IID)
@@ -186,25 +186,25 @@ NS_DEFINE_STATIC_IID_ACCESSOR(nsIDocumentObserver, NS_IDOCUMENT_OBSERVER_IID)
                                        mozilla::EventStates aStateMask) override;
 
 #define NS_DECL_NSIDOCUMENTOBSERVER_STYLESHEETADDED                          \
-    virtual void StyleSheetAdded(mozilla::CSSStyleSheet* aStyleSheet,        \
+    virtual void StyleSheetAdded(mozilla::StyleSheetHandle aStyleSheet,      \
                                  bool aDocumentSheet) override;
 
 #define NS_DECL_NSIDOCUMENTOBSERVER_STYLESHEETREMOVED                        \
-    virtual void StyleSheetRemoved(mozilla::CSSStyleSheet* aStyleSheet,      \
+    virtual void StyleSheetRemoved(mozilla::StyleSheetHandle aStyleSheet,    \
                                    bool aDocumentSheet) override;
 
 #define NS_DECL_NSIDOCUMENTOBSERVER_STYLESHEETAPPLICABLESTATECHANGED         \
     virtual void StyleSheetApplicableStateChanged(                           \
-        mozilla::CSSStyleSheet* aStyleSheet) override;
+        mozilla::StyleSheetHandle aStyleSheet) override;
 
 #define NS_DECL_NSIDOCUMENTOBSERVER_STYLERULECHANGED                         \
-    virtual void StyleRuleChanged(mozilla::CSSStyleSheet* aStyleSheet) override;
+    virtual void StyleRuleChanged(mozilla::StyleSheetHandle aStyleSheet) override;
 
 #define NS_DECL_NSIDOCUMENTOBSERVER_STYLERULEADDED                           \
-    virtual void StyleRuleAdded(mozilla::CSSStyleSheet* aStyleSheet) override;
+    virtual void StyleRuleAdded(mozilla::StyleSheetHandle aStyleSheet) override;
 
 #define NS_DECL_NSIDOCUMENTOBSERVER_STYLERULEREMOVED                         \
-    virtual void StyleRuleRemoved(mozilla::CSSStyleSheet* aStyleSheet) override;
+    virtual void StyleRuleRemoved(mozilla::StyleSheetHandle aStyleSheet) override;
 
 #define NS_DECL_NSIDOCUMENTOBSERVER                                          \
     NS_DECL_NSIDOCUMENTOBSERVER_BEGINUPDATE                                  \
@@ -262,29 +262,29 @@ NS_IMPL_NSIMUTATIONOBSERVER_CONTENT(_class)
 
 #define NS_IMPL_NSIDOCUMENTOBSERVER_STYLE_STUB(_class)                    \
 void                                                                      \
-_class::StyleSheetAdded(mozilla::CSSStyleSheet* aStyleSheet,              \
+_class::StyleSheetAdded(mozilla::StyleSheetHandle aStyleSheet,            \
                         bool aDocumentSheet)                              \
 {                                                                         \
 }                                                                         \
 void                                                                      \
-_class::StyleSheetRemoved(mozilla::CSSStyleSheet* aStyleSheet,            \
+_class::StyleSheetRemoved(mozilla::StyleSheetHandle aStyleSheet,          \
                           bool aDocumentSheet)                            \
 {                                                                         \
 }                                                                         \
 void                                                                      \
-_class::StyleSheetApplicableStateChanged(mozilla::CSSStyleSheet* aStyleSheet) \
+_class::StyleSheetApplicableStateChanged(mozilla::StyleSheetHandle aStyleSheet) \
 {                                                                         \
 }                                                                         \
 void                                                                      \
-_class::StyleRuleChanged(mozilla::CSSStyleSheet* aStyleSheet)             \
+_class::StyleRuleChanged(mozilla::StyleSheetHandle aStyleSheet)           \
 {                                                                         \
 }                                                                         \
 void                                                                      \
-_class::StyleRuleAdded(mozilla::CSSStyleSheet* aStyleSheet)               \
+_class::StyleRuleAdded(mozilla::StyleSheetHandle aStyleSheet)             \
 {                                                                         \
 }                                                                         \
 void                                                                      \
-_class::StyleRuleRemoved(mozilla::CSSStyleSheet* aStyleSheet)             \
+_class::StyleRuleRemoved(mozilla::StyleSheetHandle aStyleSheet)           \
 {                                                                         \
 }
 
diff --git a/dom/base/nsIStyleSheetLinkingElement.h b/dom/base/nsIStyleSheetLinkingElement.h
index 77eaebf6096f3..d6c2440169ba7 100644
--- a/dom/base/nsIStyleSheetLinkingElement.h
+++ b/dom/base/nsIStyleSheetLinkingElement.h
@@ -8,6 +8,7 @@
 
 
 #include "nsISupports.h"
+#include "mozilla/StyleSheetHandle.h"
 
 class nsICSSLoaderObserver;
 class nsIURI;
@@ -16,10 +17,6 @@ class nsIURI;
 { 0xa8b79f3b, 0x9d18, 0x4f9c, \
   { 0xb1, 0xaa, 0x8c, 0x9b, 0x1b, 0xaa, 0xac, 0xad } }
 
-namespace mozilla {
-class CSSStyleSheet;
-} // namespace mozilla
-
 class nsIStyleSheetLinkingElement : public nsISupports {
 public:
   NS_DECLARE_STATIC_IID_ACCESSOR(NS_ISTYLESHEETLINKINGELEMENT_IID)
@@ -31,14 +28,14 @@ class nsIStyleSheetLinkingElement : public nsISupports {
    * @param aStyleSheet the style sheet associated with this
    *                    element.
    */
-  NS_IMETHOD SetStyleSheet(mozilla::CSSStyleSheet* aStyleSheet) = 0;
+  NS_IMETHOD SetStyleSheet(mozilla::StyleSheetHandle aStyleSheet) = 0;
 
   /**
    * Used to obtain the style sheet linked in by this element.
    *
    * @return the style sheet associated with this element.
    */
-  NS_IMETHOD_(mozilla::CSSStyleSheet*) GetStyleSheet() = 0;
+  NS_IMETHOD_(mozilla::StyleSheetHandle) GetStyleSheet() = 0;
 
   /**
    * Initialize the stylesheet linking element. If aDontLoadStyle is
diff --git a/dom/base/nsStyleLinkElement.cpp b/dom/base/nsStyleLinkElement.cpp
index bf7d9bfbd9ea2..9b733ebc33f9d 100644
--- a/dom/base/nsStyleLinkElement.cpp
+++ b/dom/base/nsStyleLinkElement.cpp
@@ -12,7 +12,8 @@
 
 #include "nsStyleLinkElement.h"
 
-#include "mozilla/CSSStyleSheet.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 #include "mozilla/css/Loader.h"
 #include "mozilla/dom/Element.h"
 #include "mozilla/dom/FragmentOrElement.h"
@@ -67,7 +68,7 @@ nsStyleLinkElement::Traverse(nsCycleCollectionTraversalCallback &cb)
 }
 
 NS_IMETHODIMP 
-nsStyleLinkElement::SetStyleSheet(CSSStyleSheet* aStyleSheet)
+nsStyleLinkElement::SetStyleSheet(StyleSheetHandle aStyleSheet)
 {
   if (mStyleSheet) {
     mStyleSheet->SetOwningNode(nullptr);
@@ -84,7 +85,7 @@ nsStyleLinkElement::SetStyleSheet(CSSStyleSheet* aStyleSheet)
   return NS_OK;
 }
 
-NS_IMETHODIMP_(CSSStyleSheet*)
+NS_IMETHODIMP_(StyleSheetHandle)
 nsStyleLinkElement::GetStyleSheet()
 {
   return mStyleSheet;
@@ -316,8 +317,15 @@ nsStyleLinkElement::DoUpdateStyleSheet(nsIDocument* aOldDocument,
     return NS_OK;
   }
 
-  Element* oldScopeElement =
-    mStyleSheet ? mStyleSheet->GetScopeElement() : nullptr;
+  // XXXheycam ServoStyleSheets do not support <style scoped>.
+  Element* oldScopeElement = nullptr;
+  if (mStyleSheet) {
+    if (mStyleSheet->IsServo()) {
+      NS_ERROR("stylo: ServoStyleSheets don't support <style scoped>");
+    } else {
+      oldScopeElement = mStyleSheet->AsGecko()->GetScopeElement();
+    }
+  }
 
   if (mStyleSheet && (aOldDocument || aOldShadowRoot)) {
     MOZ_ASSERT(!(aOldDocument && aOldShadowRoot),
@@ -465,10 +473,18 @@ nsStyleLinkElement::UpdateStyleSheetScopedness(bool aIsNowScoped)
     return;
   }
 
+  if (mStyleSheet->IsServo()) {
+    // XXXheycam ServoStyleSheets don't support <style scoped>.
+    NS_ERROR("stylo: ServoStyleSheets don't support <style scoped>");
+    return;
+  }
+
+  CSSStyleSheet* sheet = mStyleSheet->AsGecko();
+
   nsCOMPtr<nsIContent> thisContent;
   CallQueryInterface(this, getter_AddRefs(thisContent));
 
-  Element* oldScopeElement = mStyleSheet->GetScopeElement();
+  Element* oldScopeElement = sheet->GetScopeElement();
   Element* newScopeElement = aIsNowScoped ?
                                thisContent->GetParentElement() :
                                nullptr;
@@ -483,14 +499,14 @@ nsStyleLinkElement::UpdateStyleSheetScopedness(bool aIsNowScoped)
     ShadowRoot* containingShadow = thisContent->GetContainingShadow();
     containingShadow->RemoveSheet(mStyleSheet);
 
-    mStyleSheet->SetScopeElement(newScopeElement);
+    sheet->SetScopeElement(newScopeElement);
 
     containingShadow->InsertSheet(mStyleSheet, thisContent);
   } else {
     document->BeginUpdate(UPDATE_STYLE);
     document->RemoveStyleSheet(mStyleSheet);
 
-    mStyleSheet->SetScopeElement(newScopeElement);
+    sheet->SetScopeElement(newScopeElement);
 
     document->AddStyleSheet(mStyleSheet);
     document->EndUpdate(UPDATE_STYLE);
diff --git a/dom/base/nsStyleLinkElement.h b/dom/base/nsStyleLinkElement.h
index a712cff8d9191..a948f099e0b7a 100644
--- a/dom/base/nsStyleLinkElement.h
+++ b/dom/base/nsStyleLinkElement.h
@@ -24,6 +24,7 @@ class nsIDocument;
 class nsIURI;
 
 namespace mozilla {
+class CSSStyleSheet;
 namespace dom {
 class ShadowRoot;
 } // namespace dom
@@ -37,11 +38,19 @@ class nsStyleLinkElement : public nsIStyleSheetLinkingElement
 
   NS_IMETHOD QueryInterface(REFNSIID aIID, void** aInstancePtr) override = 0;
 
-  mozilla::CSSStyleSheet* GetSheet() const { return mStyleSheet; }
+  mozilla::CSSStyleSheet* GetSheet() const
+  {
+    // XXXheycam Return nullptr for ServoStyleSheets until we have a way of
+    // exposing them to script.
+    NS_ASSERTION(!mStyleSheet || mStyleSheet->IsGecko(),
+                 "stylo: ServoStyleSheets can't be exposed to script yet");
+    return mStyleSheet && mStyleSheet->IsGecko() ? mStyleSheet->AsGecko() :
+                                                   nullptr;
+  }
 
   // nsIStyleSheetLinkingElement  
-  NS_IMETHOD SetStyleSheet(mozilla::CSSStyleSheet* aStyleSheet) override;
-  NS_IMETHOD_(mozilla::CSSStyleSheet*) GetStyleSheet() override;
+  NS_IMETHOD SetStyleSheet(mozilla::StyleSheetHandle aStyleSheet) override;
+  NS_IMETHOD_(mozilla::StyleSheetHandle) GetStyleSheet() override;
   NS_IMETHOD InitStyleLinkElement(bool aDontLoadStyle) override;
   NS_IMETHOD UpdateStyleSheet(nsICSSLoaderObserver* aObserver,
                               bool* aWillNotify,
@@ -128,7 +137,7 @@ class nsStyleLinkElement : public nsIStyleSheetLinkingElement
                               bool* aIsAlternate,
                               bool aForceUpdate);
 
-  RefPtr<mozilla::CSSStyleSheet> mStyleSheet;
+  mozilla::StyleSheetHandle::RefPtr mStyleSheet;
 protected:
   bool mDontLoadStyle;
   bool mUpdatesEnabled;
diff --git a/dom/html/nsHTMLDocument.cpp b/dom/html/nsHTMLDocument.cpp
index adc808adb4eaf..e13d1d0bbf5b1 100644
--- a/dom/html/nsHTMLDocument.cpp
+++ b/dom/html/nsHTMLDocument.cpp
@@ -114,6 +114,8 @@
 #include "nsIFrame.h"
 #include "nsIContent.h"
 #include "nsLayoutStylesheetCache.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
@@ -2641,12 +2643,14 @@ nsHTMLDocument::TearingDownEditor(nsIEditor *aEditor)
     if (!presShell)
       return;
 
-    nsTArray<RefPtr<CSSStyleSheet>> agentSheets;
+    nsTArray<StyleSheetHandle::RefPtr> agentSheets;
     presShell->GetAgentStyleSheets(agentSheets);
 
-    agentSheets.RemoveElement(nsLayoutStylesheetCache::ContentEditableSheet());
+    auto cache = nsLayoutStylesheetCache::For(GetStyleBackendType());
+
+    agentSheets.RemoveElement(cache->ContentEditableSheet());
     if (oldState == eDesignMode)
-      agentSheets.RemoveElement(nsLayoutStylesheetCache::DesignModeSheet());
+      agentSheets.RemoveElement(cache->DesignModeSheet());
 
     presShell->SetAgentStyleSheets(agentSheets);
 
@@ -2780,12 +2784,13 @@ nsHTMLDocument::EditingStateChanged()
     // Before making this window editable, we need to modify UA style sheet
     // because new style may change whether focused element will be focusable
     // or not.
-    nsTArray<RefPtr<CSSStyleSheet>> agentSheets;
+    nsTArray<StyleSheetHandle::RefPtr> agentSheets;
     rv = presShell->GetAgentStyleSheets(agentSheets);
     NS_ENSURE_SUCCESS(rv, rv);
 
-    CSSStyleSheet* contentEditableSheet =
-      nsLayoutStylesheetCache::ContentEditableSheet();
+    auto cache = nsLayoutStylesheetCache::For(GetStyleBackendType());
+
+    StyleSheetHandle contentEditableSheet = cache->ContentEditableSheet();
 
     if (!agentSheets.Contains(contentEditableSheet)) {
       agentSheets.AppendElement(contentEditableSheet);
@@ -2796,8 +2801,7 @@ nsHTMLDocument::EditingStateChanged()
     // specific states on the elements.
     if (designMode) {
       // designMode is being turned on (overrides contentEditable).
-      CSSStyleSheet* designModeSheet =
-        nsLayoutStylesheetCache::DesignModeSheet();
+      StyleSheetHandle designModeSheet = cache->DesignModeSheet();
       if (!agentSheets.Contains(designModeSheet)) {
         agentSheets.AppendElement(designModeSheet);
       }
@@ -2807,7 +2811,7 @@ nsHTMLDocument::EditingStateChanged()
     }
     else if (oldState == eDesignMode) {
       // designMode is being turned off (contentEditable is still on).
-      agentSheets.RemoveElement(nsLayoutStylesheetCache::DesignModeSheet());
+      agentSheets.RemoveElement(cache->DesignModeSheet());
       updateState = true;
     }
 
diff --git a/dom/ipc/ContentChild.cpp b/dom/ipc/ContentChild.cpp
index c2e334f1b75f7..af9ffffa723f1 100644
--- a/dom/ipc/ContentChild.cpp
+++ b/dom/ipc/ContentChild.cpp
@@ -2584,7 +2584,10 @@ static void
 PreloadSlowThings()
 {
   // This fetches and creates all the built-in stylesheets.
-  nsLayoutStylesheetCache::UserContentSheet();
+  //
+  // XXXheycam In the future we might want to preload the Servo-flavoured
+  // UA sheets too, but for now that will be a waste of time.
+  nsLayoutStylesheetCache::For(StyleBackendType::Gecko)->UserContentSheet();
 
   TabChild::PreloadSlowThings();
 
diff --git a/dom/ipc/ContentParent.cpp b/dom/ipc/ContentParent.cpp
index dd4ebf40db5da..e47a450a6776a 100644
--- a/dom/ipc/ContentParent.cpp
+++ b/dom/ipc/ContentParent.cpp
@@ -185,6 +185,8 @@
 #include "nsPluginHost.h"
 #include "nsPluginTags.h"
 #include "nsIBlocklistService.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 
 #include "nsIBidiKeyboard.h"
 
@@ -2697,19 +2699,19 @@ ContentParent::InitInternal(ProcessPriority aInitialPriority,
     // This looks like a lot of work, but in a normal browser session we just
     // send two loads.
 
-    for (CSSStyleSheet* sheet : *sheetService->AgentStyleSheets()) {
+    for (StyleSheetHandle sheet : *sheetService->AgentStyleSheets()) {
       URIParams uri;
       SerializeURI(sheet->GetSheetURI(), uri);
       Unused << SendLoadAndRegisterSheet(uri, nsIStyleSheetService::AGENT_SHEET);
     }
 
-    for (CSSStyleSheet* sheet : *sheetService->UserStyleSheets()) {
+    for (StyleSheetHandle sheet : *sheetService->UserStyleSheets()) {
       URIParams uri;
       SerializeURI(sheet->GetSheetURI(), uri);
       Unused << SendLoadAndRegisterSheet(uri, nsIStyleSheetService::USER_SHEET);
     }
 
-    for (CSSStyleSheet* sheet : *sheetService->AuthorStyleSheets()) {
+    for (StyleSheetHandle sheet : *sheetService->AuthorStyleSheets()) {
       URIParams uri;
       SerializeURI(sheet->GetSheetURI(), uri);
       Unused << SendLoadAndRegisterSheet(uri, nsIStyleSheetService::AUTHOR_SHEET);
diff --git a/dom/mathml/nsMathMLElement.cpp b/dom/mathml/nsMathMLElement.cpp
index d0e3aed5b81b5..bfe82749bc688 100644
--- a/dom/mathml/nsMathMLElement.cpp
+++ b/dom/mathml/nsMathMLElement.cpp
@@ -111,9 +111,9 @@ nsMathMLElement::BindToTree(nsIDocument* aDocument, nsIContent* aParent,
       // Enable MathML and setup the style sheet during binding, not element
       // construction, because we could move a MathML element from the document
       // that created it to another document.
+      auto cache = nsLayoutStylesheetCache::For(doc->GetStyleBackendType());
       doc->SetMathMLEnabled();
-      doc->
-        EnsureOnDemandBuiltInUASheet(nsLayoutStylesheetCache::MathMLSheet());
+      doc->EnsureOnDemandBuiltInUASheet(cache->MathMLSheet());
 
       // Rebuild style data for the presshell, because style system
       // optimizations may have taken place assuming MathML was disabled.
diff --git a/dom/svg/SVGDocument.cpp b/dom/svg/SVGDocument.cpp
index 84e8e3bb91910..03e6cd35c42a8 100644
--- a/dom/svg/SVGDocument.cpp
+++ b/dom/svg/SVGDocument.cpp
@@ -20,6 +20,8 @@
 #include "mozilla/dom/Element.h"
 #include "nsSVGElement.h"
 #include "mozilla/dom/SVGDocumentBinding.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 
 using namespace mozilla::css;
 using namespace mozilla::dom;
@@ -155,12 +157,12 @@ SVGDocument::EnsureNonSVGUserAgentStyleSheetsLoaded()
             nsCOMPtr<nsIURI> uri;
             NS_NewURI(getter_AddRefs(uri), spec);
             if (uri) {
-              RefPtr<CSSStyleSheet> cssSheet;
+              StyleSheetHandle::RefPtr sheet;
               cssLoader->LoadSheetSync(uri,
                                        mozilla::css::eAgentSheetFeatures,
-                                       true, getter_AddRefs(cssSheet));
-              if (cssSheet) {
-                EnsureOnDemandBuiltInUASheet(cssSheet);
+                                       true, &sheet);
+              if (sheet) {
+                EnsureOnDemandBuiltInUASheet(sheet);
               }
             }
           }
@@ -169,21 +171,23 @@ SVGDocument::EnsureNonSVGUserAgentStyleSheetsLoaded()
     }
   }
 
-  CSSStyleSheet* sheet = nsLayoutStylesheetCache::NumberControlSheet();
+  auto cache = nsLayoutStylesheetCache::For(GetStyleBackendType());
+
+  StyleSheetHandle sheet = cache->NumberControlSheet();
   if (sheet) {
     // number-control.css can be behind a pref
     EnsureOnDemandBuiltInUASheet(sheet);
   }
-  EnsureOnDemandBuiltInUASheet(nsLayoutStylesheetCache::FormsSheet());
-  EnsureOnDemandBuiltInUASheet(nsLayoutStylesheetCache::CounterStylesSheet());
-  EnsureOnDemandBuiltInUASheet(nsLayoutStylesheetCache::HTMLSheet());
+  EnsureOnDemandBuiltInUASheet(cache->FormsSheet());
+  EnsureOnDemandBuiltInUASheet(cache->CounterStylesSheet());
+  EnsureOnDemandBuiltInUASheet(cache->HTMLSheet());
   if (nsLayoutUtils::ShouldUseNoFramesSheet(this)) {
-    EnsureOnDemandBuiltInUASheet(nsLayoutStylesheetCache::NoFramesSheet());
+    EnsureOnDemandBuiltInUASheet(cache->NoFramesSheet());
   }
   if (nsLayoutUtils::ShouldUseNoScriptSheet(this)) {
-    EnsureOnDemandBuiltInUASheet(nsLayoutStylesheetCache::NoScriptSheet());
+    EnsureOnDemandBuiltInUASheet(cache->NoScriptSheet());
   }
-  EnsureOnDemandBuiltInUASheet(nsLayoutStylesheetCache::UASheet());
+  EnsureOnDemandBuiltInUASheet(cache->UASheet());
 
   EndUpdate(UPDATE_STYLE);
 }
diff --git a/dom/svg/SVGSVGElement.cpp b/dom/svg/SVGSVGElement.cpp
index 0bf009865e97b..df60744509fe4 100644
--- a/dom/svg/SVGSVGElement.cpp
+++ b/dom/svg/SVGSVGElement.cpp
@@ -752,7 +752,8 @@ SVGSVGElement::BindToTree(nsIDocument* aDocument,
     // Setup the style sheet during binding, not element construction,
     // because we could move the root SVG element from the document
     // that created it to another document.
-    doc->EnsureOnDemandBuiltInUASheet(nsLayoutStylesheetCache::SVGSheet());
+    auto cache = nsLayoutStylesheetCache::For(doc->GetStyleBackendType());
+    doc->EnsureOnDemandBuiltInUASheet(cache->SVGSheet());
   }
 
   if (mTimedDocumentRoot && smilController) {
diff --git a/dom/xbl/nsBindingManager.cpp b/dom/xbl/nsBindingManager.cpp
index 898f5dd3bda0b..3e4dd8ea041d0 100644
--- a/dom/xbl/nsBindingManager.cpp
+++ b/dom/xbl/nsBindingManager.cpp
@@ -773,7 +773,7 @@ nsBindingManager::MediumFeaturesChanged(nsPresContext* aPresContext,
 }
 
 void
-nsBindingManager::AppendAllSheets(nsTArray<CSSStyleSheet*>& aArray)
+nsBindingManager::AppendAllSheets(nsTArray<StyleSheetHandle>& aArray)
 {
   if (!mBoundContentSet) {
     return;
diff --git a/dom/xbl/nsBindingManager.h b/dom/xbl/nsBindingManager.h
index 1a6ee8ad634d7..26272974e9423 100644
--- a/dom/xbl/nsBindingManager.h
+++ b/dom/xbl/nsBindingManager.h
@@ -17,6 +17,7 @@
 #include "nsXBLBinding.h"
 #include "nsTArray.h"
 #include "nsThreadUtils.h"
+#include "mozilla/StyleSheetHandle.h"
 
 struct ElementDependentRuleProcessorData;
 class nsIXPConnectWrappedJS;
@@ -31,10 +32,6 @@ typedef nsTArray<RefPtr<nsXBLBinding> > nsBindingList;
 class nsIPrincipal;
 class nsITimer;
 
-namespace mozilla {
-class CSSStyleSheet;
-} // namespace mozilla
-
 class nsBindingManager final : public nsStubMutationObserver
 {
   ~nsBindingManager();
@@ -129,7 +126,7 @@ class nsBindingManager final : public nsStubMutationObserver
   nsresult MediumFeaturesChanged(nsPresContext* aPresContext,
                                  bool* aRulesChanged);
 
-  void AppendAllSheets(nsTArray<mozilla::CSSStyleSheet*>& aArray);
+  void AppendAllSheets(nsTArray<mozilla::StyleSheetHandle>& aArray);
 
   void Traverse(nsIContent *aContent,
                             nsCycleCollectionTraversalCallback &cb);
diff --git a/dom/xbl/nsXBLPrototypeBinding.cpp b/dom/xbl/nsXBLPrototypeBinding.cpp
index 5577d5537bfdc..096ace4ba1c85 100644
--- a/dom/xbl/nsXBLPrototypeBinding.cpp
+++ b/dom/xbl/nsXBLPrototypeBinding.cpp
@@ -44,6 +44,8 @@
 #include "mozilla/dom/CDATASection.h"
 #include "mozilla/dom/Comment.h"
 #include "mozilla/dom/Element.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 
 #ifdef MOZ_XUL
 #include "nsXULElement.h"
@@ -1630,14 +1632,14 @@ nsXBLPrototypeBinding::EnsureResources()
 }
 
 void
-nsXBLPrototypeBinding::AppendStyleSheet(CSSStyleSheet* aSheet)
+nsXBLPrototypeBinding::AppendStyleSheet(StyleSheetHandle aSheet)
 {
   EnsureResources();
   mResources->AppendStyleSheet(aSheet);
 }
 
 void
-nsXBLPrototypeBinding::RemoveStyleSheet(CSSStyleSheet* aSheet)
+nsXBLPrototypeBinding::RemoveStyleSheet(StyleSheetHandle aSheet)
 {
   if (!mResources) {
     MOZ_ASSERT(false, "Trying to remove a sheet that does not exist.");
@@ -1647,13 +1649,13 @@ nsXBLPrototypeBinding::RemoveStyleSheet(CSSStyleSheet* aSheet)
   mResources->RemoveStyleSheet(aSheet);
 } 
 void
-nsXBLPrototypeBinding::InsertStyleSheetAt(size_t aIndex, CSSStyleSheet* aSheet)
+nsXBLPrototypeBinding::InsertStyleSheetAt(size_t aIndex, StyleSheetHandle aSheet)
 {
   EnsureResources();
   mResources->InsertStyleSheetAt(aIndex, aSheet);
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsXBLPrototypeBinding::StyleSheetAt(size_t aIndex) const
 {
   MOZ_ASSERT(mResources);
@@ -1674,7 +1676,7 @@ nsXBLPrototypeBinding::HasStyleSheets() const
 
 void
 nsXBLPrototypeBinding::AppendStyleSheetsTo(
-                                      nsTArray<CSSStyleSheet*>& aResult) const
+                                      nsTArray<StyleSheetHandle>& aResult) const
 {
   if (mResources) {
     mResources->AppendStyleSheetsTo(aResult);
diff --git a/dom/xbl/nsXBLPrototypeBinding.h b/dom/xbl/nsXBLPrototypeBinding.h
index c7bc36a2a3a20..c6c870be6c3f1 100644
--- a/dom/xbl/nsXBLPrototypeBinding.h
+++ b/dom/xbl/nsXBLPrototypeBinding.h
@@ -19,6 +19,7 @@
 #include "nsXBLPrototypeHandler.h"
 #include "nsXBLPrototypeResources.h"
 #include "mozilla/WeakPtr.h"
+#include "mozilla/StyleSheetHandle.h"
 
 class nsIAtom;
 class nsIContent;
@@ -27,10 +28,6 @@ class nsXBLAttributeEntry;
 class nsXBLBinding;
 class nsXBLProtoImplField;
 
-namespace mozilla {
-class CSSStyleSheet;
-} // namespace mozilla
-
 // *********************************************************************/
 // The XBLPrototypeBinding class
 
@@ -122,13 +119,13 @@ class nsXBLPrototypeBinding final :
 
   void SetInitialAttributes(nsIContent* aBoundElement, nsIContent* aAnonymousContent);
 
-  void AppendStyleSheet(mozilla::CSSStyleSheet* aSheet);
-  void RemoveStyleSheet(mozilla::CSSStyleSheet* aSheet);
-  void InsertStyleSheetAt(size_t aIndex, mozilla::CSSStyleSheet* aSheet);
-  mozilla::CSSStyleSheet* StyleSheetAt(size_t aIndex) const;
+  void AppendStyleSheet(mozilla::StyleSheetHandle aSheet);
+  void RemoveStyleSheet(mozilla::StyleSheetHandle aSheet);
+  void InsertStyleSheetAt(size_t aIndex, mozilla::StyleSheetHandle aSheet);
+  mozilla::StyleSheetHandle StyleSheetAt(size_t aIndex) const;
   size_t SheetCount() const;
   bool HasStyleSheets() const;
-  void AppendStyleSheetsTo(nsTArray<mozilla::CSSStyleSheet*>& aResult) const;
+  void AppendStyleSheetsTo(nsTArray<mozilla::StyleSheetHandle>& aResult) const;
 
   nsIStyleRuleProcessor* GetRuleProcessor();
 
diff --git a/dom/xbl/nsXBLPrototypeResources.cpp b/dom/xbl/nsXBLPrototypeResources.cpp
index cd1cb192d14d7..10a9f7494b006 100644
--- a/dom/xbl/nsXBLPrototypeResources.cpp
+++ b/dom/xbl/nsXBLPrototypeResources.cpp
@@ -19,6 +19,8 @@
 #include "nsStyleSet.h"
 #include "mozilla/dom/URL.h"
 #include "mozilla/DebugOnly.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 
 using namespace mozilla;
 using mozilla::dom::IsChromeURI;
@@ -80,20 +82,20 @@ nsXBLPrototypeResources::FlushSkinSheets()
   // encounter.  (If they aren't skin sheets, it doesn't matter, since
   // they'll still be in the chrome cache.
 
-  nsTArray<RefPtr<CSSStyleSheet>> oldSheets;
+  nsTArray<StyleSheetHandle::RefPtr> oldSheets;
 
   oldSheets.SwapElements(mStyleSheetList);
 
   mozilla::css::Loader* cssLoader = doc->CSSLoader();
 
   for (size_t i = 0, count = oldSheets.Length(); i < count; ++i) {
-    CSSStyleSheet* oldSheet = oldSheets[i];
+    StyleSheetHandle oldSheet = oldSheets[i];
 
     nsIURI* uri = oldSheet->GetSheetURI();
 
-    RefPtr<CSSStyleSheet> newSheet;
+    StyleSheetHandle::RefPtr newSheet;
     if (IsChromeURI(uri)) {
-      if (NS_FAILED(cssLoader->LoadSheetSync(uri, getter_AddRefs(newSheet))))
+      if (NS_FAILED(cssLoader->LoadSheetSync(uri, &newSheet)))
         continue;
     }
     else {
@@ -142,31 +144,39 @@ nsXBLPrototypeResources::ClearLoader()
 void
 nsXBLPrototypeResources::GatherRuleProcessor()
 {
-  mRuleProcessor = new nsCSSRuleProcessor(mStyleSheetList,
+  nsTArray<RefPtr<CSSStyleSheet>> sheets(mStyleSheetList.Length());
+  for (StyleSheetHandle sheet : mStyleSheetList) {
+    MOZ_ASSERT(sheet->IsGecko(),
+               "GatherRuleProcessor must only be called for "
+               "nsXBLPrototypeResources objects with Gecko-flavored style "
+               "backends");
+    sheets.AppendElement(sheet->AsGecko());
+  }
+  mRuleProcessor = new nsCSSRuleProcessor(sheets,
                                           SheetType::Doc,
                                           nullptr,
                                           mRuleProcessor);
 }
 
 void
-nsXBLPrototypeResources::AppendStyleSheet(CSSStyleSheet* aSheet)
+nsXBLPrototypeResources::AppendStyleSheet(StyleSheetHandle aSheet)
 {
   mStyleSheetList.AppendElement(aSheet);
 }
 
 void
-nsXBLPrototypeResources::RemoveStyleSheet(CSSStyleSheet* aSheet)
+nsXBLPrototypeResources::RemoveStyleSheet(StyleSheetHandle aSheet)
 {
   mStyleSheetList.RemoveElement(aSheet);
 }
 
 void
-nsXBLPrototypeResources::InsertStyleSheetAt(size_t aIndex, CSSStyleSheet* aSheet)
+nsXBLPrototypeResources::InsertStyleSheetAt(size_t aIndex, StyleSheetHandle aSheet)
 {
   mStyleSheetList.InsertElementAt(aIndex, aSheet);
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsXBLPrototypeResources::StyleSheetAt(size_t aIndex) const
 {
   return mStyleSheetList[aIndex];
@@ -186,7 +196,7 @@ nsXBLPrototypeResources::HasStyleSheets() const
 
 void
 nsXBLPrototypeResources::AppendStyleSheetsTo(
-                                      nsTArray<CSSStyleSheet*>& aResult) const
+                                      nsTArray<StyleSheetHandle>& aResult) const
 {
   aResult.AppendElements(mStyleSheetList);
 }
diff --git a/dom/xbl/nsXBLPrototypeResources.h b/dom/xbl/nsXBLPrototypeResources.h
index 4305a09e25138..f6d67bdcffa51 100644
--- a/dom/xbl/nsXBLPrototypeResources.h
+++ b/dom/xbl/nsXBLPrototypeResources.h
@@ -7,6 +7,7 @@
 #ifndef nsXBLPrototypeResources_h__
 #define nsXBLPrototypeResources_h__
 
+#include "mozilla/StyleSheetHandle.h"
 #include "nsAutoPtr.h"
 #include "nsICSSLoaderObserver.h"
 
@@ -41,13 +42,13 @@ class nsXBLPrototypeResources
 
   void ClearLoader();
 
-  void AppendStyleSheet(mozilla::CSSStyleSheet* aSheet);
-  void RemoveStyleSheet(mozilla::CSSStyleSheet* aSheet);
-  void InsertStyleSheetAt(size_t aIndex, mozilla::CSSStyleSheet* aSheet);
-  mozilla::CSSStyleSheet* StyleSheetAt(size_t aIndex) const;
+  void AppendStyleSheet(mozilla::StyleSheetHandle aSheet);
+  void RemoveStyleSheet(mozilla::StyleSheetHandle aSheet);
+  void InsertStyleSheetAt(size_t aIndex, mozilla::StyleSheetHandle aSheet);
+  mozilla::StyleSheetHandle StyleSheetAt(size_t aIndex) const;
   size_t SheetCount() const;
   bool HasStyleSheets() const;
-  void AppendStyleSheetsTo(nsTArray<mozilla::CSSStyleSheet*>& aResult) const;
+  void AppendStyleSheetsTo(nsTArray<mozilla::StyleSheetHandle>& aResult) const;
 
   /**
    * Recreates mRuleProcessor to represent the current list of style sheets
@@ -63,7 +64,7 @@ class nsXBLPrototypeResources
   RefPtr<nsXBLResourceLoader> mLoader;
 
   // A list of loaded stylesheets for this binding.
-  nsTArray<RefPtr<mozilla::CSSStyleSheet>> mStyleSheetList;
+  nsTArray<mozilla::StyleSheetHandle::RefPtr> mStyleSheetList;
 
   // The list of stylesheets converted to a rule processor.
   RefPtr<nsCSSRuleProcessor> mRuleProcessor;
diff --git a/dom/xbl/nsXBLResourceLoader.cpp b/dom/xbl/nsXBLResourceLoader.cpp
index 96e397d0db19d..7c15b6fa0ced1 100644
--- a/dom/xbl/nsXBLResourceLoader.cpp
+++ b/dom/xbl/nsXBLResourceLoader.cpp
@@ -17,7 +17,8 @@
 #include "nsIDocumentObserver.h"
 #include "imgILoader.h"
 #include "imgRequestProxy.h"
-#include "mozilla/CSSStyleSheet.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 #include "mozilla/css/Loader.h"
 #include "nsIURI.h"
 #include "nsNetUtil.h"
@@ -139,8 +140,8 @@ nsXBLResourceLoader::LoadResources(bool* aResult)
           CheckLoadURIWithPrincipal(docPrincipal, url,
                                     nsIScriptSecurityManager::ALLOW_CHROME);
         if (NS_SUCCEEDED(rv)) {
-          RefPtr<CSSStyleSheet> sheet;
-          rv = cssLoader->LoadSheetSync(url, getter_AddRefs(sheet));
+          StyleSheetHandle::RefPtr sheet;
+          rv = cssLoader->LoadSheetSync(url, &sheet);
           NS_ASSERTION(NS_SUCCEEDED(rv), "Load failed!!!");
           if (NS_SUCCEEDED(rv))
           {
@@ -168,7 +169,7 @@ nsXBLResourceLoader::LoadResources(bool* aResult)
 
 // nsICSSLoaderObserver
 NS_IMETHODIMP
-nsXBLResourceLoader::StyleSheetLoaded(CSSStyleSheet* aSheet,
+nsXBLResourceLoader::StyleSheetLoaded(StyleSheetHandle aSheet,
                                       bool aWasAlternate,
                                       nsresult aStatus)
 {
diff --git a/dom/xbl/nsXBLResourceLoader.h b/dom/xbl/nsXBLResourceLoader.h
index cbe59a74c077b..17b2f5c2e5d1d 100644
--- a/dom/xbl/nsXBLResourceLoader.h
+++ b/dom/xbl/nsXBLResourceLoader.h
@@ -30,7 +30,7 @@ class nsXBLResourceLoader : public nsICSSLoaderObserver
   NS_DECL_CYCLE_COLLECTION_CLASS(nsXBLResourceLoader)
 
   // nsICSSLoaderObserver
-  NS_IMETHOD StyleSheetLoaded(mozilla::CSSStyleSheet* aSheet,
+  NS_IMETHOD StyleSheetLoaded(mozilla::StyleSheetHandle aSheet,
                               bool aWasAlternate,
                               nsresult aStatus) override;
 
diff --git a/dom/xml/nsXMLContentSink.cpp b/dom/xml/nsXMLContentSink.cpp
index ba209234ee79f..5ec56314da185 100644
--- a/dom/xml/nsXMLContentSink.cpp
+++ b/dom/xml/nsXMLContentSink.cpp
@@ -411,7 +411,7 @@ nsXMLContentSink::OnTransformDone(nsresult aResult,
 }
 
 NS_IMETHODIMP
-nsXMLContentSink::StyleSheetLoaded(CSSStyleSheet* aSheet,
+nsXMLContentSink::StyleSheetLoaded(StyleSheetHandle aSheet,
                                    bool aWasAlternate,
                                    nsresult aStatus)
 {
diff --git a/dom/xml/nsXMLContentSink.h b/dom/xml/nsXMLContentSink.h
index 953232d044fe3..fbdac6633eff9 100644
--- a/dom/xml/nsXMLContentSink.h
+++ b/dom/xml/nsXMLContentSink.h
@@ -82,7 +82,7 @@ class nsXMLContentSink : public nsContentSink,
   NS_IMETHOD OnTransformDone(nsresult aResult, nsIDocument *aResultDocument) override;
 
   // nsICSSLoaderObserver
-  NS_IMETHOD StyleSheetLoaded(mozilla::CSSStyleSheet* aSheet,
+  NS_IMETHOD StyleSheetLoaded(mozilla::StyleSheetHandle aSheet,
                               bool aWasAlternate,
                               nsresult aStatus) override;
   static bool ParsePIData(const nsString &aData, nsString &aHref,
diff --git a/dom/xslt/xslt/txMozillaXMLOutput.cpp b/dom/xslt/xslt/txMozillaXMLOutput.cpp
index 18230d5b7ac81..061d7bf3e32ed 100644
--- a/dom/xslt/xslt/txMozillaXMLOutput.cpp
+++ b/dom/xslt/xslt/txMozillaXMLOutput.cpp
@@ -976,7 +976,7 @@ txTransformNotifier::ScriptEvaluated(nsresult aResult,
 }
 
 NS_IMETHODIMP 
-txTransformNotifier::StyleSheetLoaded(CSSStyleSheet* aSheet,
+txTransformNotifier::StyleSheetLoaded(StyleSheetHandle aSheet,
                                       bool aWasAlternate,
                                       nsresult aStatus)
 {
diff --git a/dom/xslt/xslt/txMozillaXMLOutput.h b/dom/xslt/xslt/txMozillaXMLOutput.h
index a54425b9f1d34..a4897302bdef5 100644
--- a/dom/xslt/xslt/txMozillaXMLOutput.h
+++ b/dom/xslt/xslt/txMozillaXMLOutput.h
@@ -35,7 +35,7 @@ class txTransformNotifier final : public nsIScriptLoaderObserver,
     NS_DECL_NSISCRIPTLOADEROBSERVER
     
     // nsICSSLoaderObserver
-    NS_IMETHOD StyleSheetLoaded(mozilla::CSSStyleSheet* aSheet,
+    NS_IMETHOD StyleSheetLoaded(mozilla::StyleSheetHandle aSheet,
                                 bool aWasAlternate,
                                 nsresult aStatus) override;
 
diff --git a/dom/xul/XULDocument.cpp b/dom/xul/XULDocument.cpp
index e61489c8420dc..2c33e7b34379d 100644
--- a/dom/xul/XULDocument.cpp
+++ b/dom/xul/XULDocument.cpp
@@ -93,6 +93,8 @@
 #include "mozilla/dom/URL.h"
 #include "nsIContentPolicy.h"
 #include "mozAutoDocUpdate.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
@@ -3098,7 +3100,7 @@ XULDocument::DoneWalking()
 }
 
 NS_IMETHODIMP
-XULDocument::StyleSheetLoaded(CSSStyleSheet* aSheet,
+XULDocument::StyleSheetLoaded(StyleSheetHandle aSheet,
                               bool aWasAlternate,
                               nsresult aStatus)
 {
@@ -3727,11 +3729,11 @@ XULDocument::AddPrototypeSheets()
     for (int32_t i = 0; i < sheets.Count(); i++) {
         nsCOMPtr<nsIURI> uri = sheets[i];
 
-        RefPtr<CSSStyleSheet> incompleteSheet;
+        StyleSheetHandle::RefPtr incompleteSheet;
         rv = CSSLoader()->LoadSheet(uri,
                                     mCurrentPrototype->DocumentPrincipal(),
                                     EmptyCString(), this,
-                                    getter_AddRefs(incompleteSheet));
+                                    &incompleteSheet);
 
         // XXXldb We need to prevent bogus sheets from being held in the
         // prototype's list, but until then, don't propagate the failure
diff --git a/dom/xul/XULDocument.h b/dom/xul/XULDocument.h
index 397057b297302..e77143de23b86 100644
--- a/dom/xul/XULDocument.h
+++ b/dom/xul/XULDocument.h
@@ -12,6 +12,7 @@
 #include "nsTArray.h"
 
 #include "mozilla/dom/XMLDocument.h"
+#include "mozilla/StyleSheetHandle.h"
 #include "nsForwardReference.h"
 #include "nsIContent.h"
 #include "nsIDOMXULCommandDispatcher.h"
@@ -161,7 +162,7 @@ class XULDocument final : public XMLDocument,
     NS_DECL_NSIDOMXULDOCUMENT
 
     // nsICSSLoaderObserver
-    NS_IMETHOD StyleSheetLoaded(CSSStyleSheet* aSheet,
+    NS_IMETHOD StyleSheetLoaded(mozilla::StyleSheetHandle aSheet,
                                 bool aWasAlternate,
                                 nsresult aStatus) override;
 
@@ -342,7 +343,7 @@ class XULDocument final : public XMLDocument,
      * An array of style sheets, that will be added (preserving order) to the
      * document after all of them are loaded (in DoneWalking).
      */
-    nsTArray<RefPtr<CSSStyleSheet>> mOverlaySheets;
+    nsTArray<StyleSheetHandle::RefPtr> mOverlaySheets;
 
     nsCOMPtr<nsIDOMXULCommandDispatcher>     mCommandDispatcher; // [OWNER] of the focus tracker
 
diff --git a/dom/xul/nsXULElement.cpp b/dom/xul/nsXULElement.cpp
index 4899c2f6dc80c..9d45eb5355073 100644
--- a/dom/xul/nsXULElement.cpp
+++ b/dom/xul/nsXULElement.cpp
@@ -859,7 +859,8 @@ nsXULElement::BindToTree(nsIDocument* aDocument,
     // can be moved from the document that creates them to another document.
 
     if (!XULElementsRulesInMinimalXULSheet(NodeInfo()->NameAtom())) {
-      doc->EnsureOnDemandBuiltInUASheet(nsLayoutStylesheetCache::XULSheet());
+      auto cache = nsLayoutStylesheetCache::For(doc->GetStyleBackendType());
+      doc->EnsureOnDemandBuiltInUASheet(cache->XULSheet());
       // To keep memory usage down it is important that we try and avoid
       // pulling xul.css into non-XUL documents. That should be very rare, and
       // for HTML we currently should only pull it in if the document contains
diff --git a/editor/libeditor/nsEditor.cpp b/editor/libeditor/nsEditor.cpp
index 160f7009d4c61..c357071f60afa 100644
--- a/editor/libeditor/nsEditor.cpp
+++ b/editor/libeditor/nsEditor.cpp
@@ -4253,7 +4253,7 @@ nsEditor::CreateTxnForIMEText(const nsAString& aStringToInsert)
 
 
 NS_IMETHODIMP
-nsEditor::CreateTxnForAddStyleSheet(CSSStyleSheet* aSheet, AddStyleSheetTxn* *aTxn)
+nsEditor::CreateTxnForAddStyleSheet(StyleSheetHandle aSheet, AddStyleSheetTxn* *aTxn)
 {
   RefPtr<AddStyleSheetTxn> txn = new AddStyleSheetTxn();
 
@@ -4269,7 +4269,7 @@ nsEditor::CreateTxnForAddStyleSheet(CSSStyleSheet* aSheet, AddStyleSheetTxn* *aT
 
 
 NS_IMETHODIMP
-nsEditor::CreateTxnForRemoveStyleSheet(CSSStyleSheet* aSheet, RemoveStyleSheetTxn* *aTxn)
+nsEditor::CreateTxnForRemoveStyleSheet(StyleSheetHandle aSheet, RemoveStyleSheetTxn* *aTxn)
 {
   RefPtr<RemoveStyleSheetTxn> txn = new RemoveStyleSheetTxn();
 
diff --git a/editor/libeditor/nsEditor.h b/editor/libeditor/nsEditor.h
index 725898ece171e..ad6b2c75d0b9c 100644
--- a/editor/libeditor/nsEditor.h
+++ b/editor/libeditor/nsEditor.h
@@ -8,6 +8,7 @@
 
 #include "mozilla/Assertions.h"         // for MOZ_ASSERT, etc.
 #include "mozilla/OwningNonNull.h"      // for OwningNonNull
+#include "mozilla/StyleSheetHandle.h"   // for StyleSheetHandle
 #include "mozilla/dom/Text.h"
 #include "nsAutoPtr.h"                  // for nsRefPtr
 #include "nsCOMPtr.h"                   // for already_AddRefed, nsCOMPtr
@@ -55,7 +56,6 @@ class nsTransactionManager;
 struct DOMPoint;
 
 namespace mozilla {
-class CSSStyleSheet;
 class ErrorResult;
 class TextComposition;
 
@@ -311,12 +311,12 @@ class nsEditor : public nsIEditor,
 
   /** create a transaction for adding a style sheet
     */
-  NS_IMETHOD CreateTxnForAddStyleSheet(mozilla::CSSStyleSheet* aSheet,
+  NS_IMETHOD CreateTxnForAddStyleSheet(mozilla::StyleSheetHandle aSheet,
                                        AddStyleSheetTxn* *aTxn);
 
   /** create a transaction for removing a style sheet
     */
-  NS_IMETHOD CreateTxnForRemoveStyleSheet(mozilla::CSSStyleSheet* aSheet,
+  NS_IMETHOD CreateTxnForRemoveStyleSheet(mozilla::StyleSheetHandle aSheet,
                                           RemoveStyleSheetTxn* *aTxn);
 
   nsresult DeleteText(nsGenericDOMDataNode& aElement,
diff --git a/editor/libeditor/nsHTMLEditor.cpp b/editor/libeditor/nsHTMLEditor.cpp
index c8c4ddfd24054..bd30cf9242dc4 100644
--- a/editor/libeditor/nsHTMLEditor.cpp
+++ b/editor/libeditor/nsHTMLEditor.cpp
@@ -37,7 +37,6 @@
 #include "nsILinkHandler.h"
 #include "nsIInlineSpellChecker.h"
 
-#include "mozilla/CSSStyleSheet.h"
 #include "mozilla/css/Loader.h"
 #include "nsIDOMStyleSheet.h"
 
@@ -75,6 +74,8 @@
 #include "mozilla/dom/HTMLBodyElement.h"
 #include "nsTextFragment.h"
 #include "nsContentList.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
@@ -2825,13 +2826,11 @@ nsHTMLEditor::ReplaceStyleSheet(const nsAString& aURL)
 NS_IMETHODIMP
 nsHTMLEditor::RemoveStyleSheet(const nsAString &aURL)
 {
-  RefPtr<CSSStyleSheet> sheet;
-  nsresult rv = GetStyleSheetForURL(aURL, getter_AddRefs(sheet));
-  NS_ENSURE_SUCCESS(rv, rv);
+  StyleSheetHandle::RefPtr sheet = GetStyleSheetForURL(aURL);
   NS_ENSURE_TRUE(sheet, NS_ERROR_UNEXPECTED);
 
   RefPtr<RemoveStyleSheetTxn> txn;
-  rv = CreateTxnForRemoveStyleSheet(sheet, getter_AddRefs(txn));
+  nsresult rv = CreateTxnForRemoveStyleSheet(sheet, getter_AddRefs(txn));
   if (!txn) rv = NS_ERROR_NULL_POINTER;
   if (NS_SUCCEEDED(rv))
   {
@@ -2865,11 +2864,11 @@ nsHTMLEditor::AddOverrideStyleSheet(const nsAString& aURL)
   // We MUST ONLY load synchronous local files (no @import)
   // XXXbz Except this will actually try to load remote files
   // synchronously, of course..
-  RefPtr<CSSStyleSheet> sheet;
+  StyleSheetHandle::RefPtr sheet;
   // Editor override style sheets may want to style Gecko anonymous boxes
   rv = ps->GetDocument()->CSSLoader()->
     LoadSheetSync(uaURI, mozilla::css::eAgentSheetFeatures, true,
-                  getter_AddRefs(sheet));
+                  &sheet);
 
   // Synchronous loads should ALWAYS return completed
   NS_ENSURE_TRUE(sheet, NS_ERROR_NULL_POINTER);
@@ -2910,8 +2909,7 @@ nsHTMLEditor::ReplaceOverrideStyleSheet(const nsAString& aURL)
 NS_IMETHODIMP
 nsHTMLEditor::RemoveOverrideStyleSheet(const nsAString &aURL)
 {
-  RefPtr<CSSStyleSheet> sheet;
-  GetStyleSheetForURL(aURL, getter_AddRefs(sheet));
+  StyleSheetHandle::RefPtr sheet = GetStyleSheetForURL(aURL);
 
   // Make sure we remove the stylesheet from our internal list in all
   // cases.
@@ -2933,24 +2931,25 @@ nsHTMLEditor::RemoveOverrideStyleSheet(const nsAString &aURL)
 NS_IMETHODIMP
 nsHTMLEditor::EnableStyleSheet(const nsAString &aURL, bool aEnable)
 {
-  RefPtr<CSSStyleSheet> sheet;
-  nsresult rv = GetStyleSheetForURL(aURL, getter_AddRefs(sheet));
-  NS_ENSURE_SUCCESS(rv, rv);
+  StyleSheetHandle::RefPtr sheet = GetStyleSheetForURL(aURL);
   NS_ENSURE_TRUE(sheet, NS_OK); // Don't fail if sheet not found
 
   // Ensure the style sheet is owned by our document.
   nsCOMPtr<nsIDocument> doc = do_QueryReferent(mDocWeak);
   sheet->SetOwningDocument(doc);
 
-  return sheet->SetDisabled(!aEnable);
+  if (sheet->IsServo()) {
+    // XXXheycam ServoStyleSheets don't support being enabled/disabled yet.
+    NS_ERROR("stylo: ServoStyleSheets can't be disabled yet");
+    return NS_ERROR_FAILURE;
+  }
+  return sheet->AsGecko()->SetDisabled(!aEnable);
 }
 
 bool
 nsHTMLEditor::EnableExistingStyleSheet(const nsAString &aURL)
 {
-  RefPtr<CSSStyleSheet> sheet;
-  nsresult rv = GetStyleSheetForURL(aURL, getter_AddRefs(sheet));
-  NS_ENSURE_SUCCESS(rv, false);
+  StyleSheetHandle::RefPtr sheet = GetStyleSheetForURL(aURL);
 
   // Enable sheet if already loaded.
   if (sheet)
@@ -2959,7 +2958,12 @@ nsHTMLEditor::EnableExistingStyleSheet(const nsAString &aURL)
     nsCOMPtr<nsIDocument> doc = do_QueryReferent(mDocWeak);
     sheet->SetOwningDocument(doc);
 
-    sheet->SetDisabled(false);
+    if (sheet->IsServo()) {
+      // XXXheycam ServoStyleSheets don't support being enabled/disabled yet.
+      NS_ERROR("stylo: ServoStyleSheets can't be disabled yet");
+      return true;
+    }
+    sheet->AsGecko()->SetDisabled(false);
     return true;
   }
   return false;
@@ -2967,7 +2971,7 @@ nsHTMLEditor::EnableExistingStyleSheet(const nsAString &aURL)
 
 nsresult
 nsHTMLEditor::AddNewStyleSheetToList(const nsAString &aURL,
-                                     CSSStyleSheet* aStyleSheet)
+                                     StyleSheetHandle aStyleSheet)
 {
   uint32_t countSS = mStyleSheets.Length();
   uint32_t countU = mStyleSheetURLs.Length();
@@ -2997,41 +3001,33 @@ nsHTMLEditor::RemoveStyleSheetFromList(const nsAString &aURL)
   return NS_OK;
 }
 
-NS_IMETHODIMP
-nsHTMLEditor::GetStyleSheetForURL(const nsAString &aURL,
-                                  CSSStyleSheet** aStyleSheet)
+StyleSheetHandle
+nsHTMLEditor::GetStyleSheetForURL(const nsAString& aURL)
 {
-  NS_ENSURE_ARG_POINTER(aStyleSheet);
-  *aStyleSheet = 0;
-
   // is it already in the list?
   size_t foundIndex;
   foundIndex = mStyleSheetURLs.IndexOf(aURL);
-  if (foundIndex == mStyleSheetURLs.NoIndex)
-    return NS_OK; //No sheet -- don't fail!
-
-  *aStyleSheet = mStyleSheets[foundIndex];
-  NS_ENSURE_TRUE(*aStyleSheet, NS_ERROR_FAILURE);
-
-  NS_ADDREF(*aStyleSheet);
+  if (foundIndex == mStyleSheetURLs.NoIndex) {
+    return nullptr;
+  }
 
-  return NS_OK;
+  MOZ_ASSERT(mStyleSheets[foundIndex]);
+  return mStyleSheets[foundIndex];
 }
 
-NS_IMETHODIMP
-nsHTMLEditor::GetURLForStyleSheet(CSSStyleSheet* aStyleSheet,
-                                  nsAString &aURL)
+void
+nsHTMLEditor::GetURLForStyleSheet(StyleSheetHandle aStyleSheet,
+                                  nsAString& aURL)
 {
   // is it already in the list?
   int32_t foundIndex = mStyleSheets.IndexOf(aStyleSheet);
 
   // Don't fail if we don't find it in our list
   if (foundIndex == -1)
-    return NS_OK;
+    return;
 
   // Found it in the list!
   aURL = mStyleSheetURLs[foundIndex];
-  return NS_OK;
 }
 
 /*
@@ -3395,7 +3391,7 @@ nsHTMLEditor::DebugUnitTests(int32_t *outNumTests, int32_t *outNumTestsFailed)
 
 
 NS_IMETHODIMP
-nsHTMLEditor::StyleSheetLoaded(CSSStyleSheet* aSheet, bool aWasAlternate,
+nsHTMLEditor::StyleSheetLoaded(StyleSheetHandle aSheet, bool aWasAlternate,
                                nsresult aStatus)
 {
   nsresult rv = NS_OK;
diff --git a/editor/libeditor/nsHTMLEditor.h b/editor/libeditor/nsHTMLEditor.h
index 9aec9de301d56..754a3b66c0826 100644
--- a/editor/libeditor/nsHTMLEditor.h
+++ b/editor/libeditor/nsHTMLEditor.h
@@ -39,6 +39,7 @@
 
 #include "mozilla/Attributes.h"
 #include "mozilla/dom/Element.h"
+#include "mozilla/StyleSheetHandle.h"
 
 class nsDocumentFragment;
 class nsIDOMKeyEvent;
@@ -333,7 +334,7 @@ class nsHTMLEditor final : public nsPlaintextEditor,
   NS_IMETHOD GetRootElement(nsIDOMElement **aRootElement) override;
 
   /* ------------ nsICSSLoaderObserver -------------- */
-  NS_IMETHOD StyleSheetLoaded(mozilla::CSSStyleSheet* aSheet,
+  NS_IMETHOD StyleSheetLoaded(mozilla::StyleSheetHandle aSheet,
                               bool aWasAlternate, nsresult aStatus) override;
 
   /* ------------ Utility Routines, not part of public API -------------- */
@@ -378,14 +379,13 @@ class nsHTMLEditor final : public nsPlaintextEditor,
   bool     EnableExistingStyleSheet(const nsAString& aURL);
 
   // Dealing with the internal style sheet lists:
-  NS_IMETHOD GetStyleSheetForURL(const nsAString &aURL,
-                                 mozilla::CSSStyleSheet** _retval) override;
-  NS_IMETHOD GetURLForStyleSheet(mozilla::CSSStyleSheet* aStyleSheet,
-                                 nsAString& aURL) override;
+  mozilla::StyleSheetHandle GetStyleSheetForURL(const nsAString& aURL);
+  void GetURLForStyleSheet(mozilla::StyleSheetHandle aStyleSheet,
+                           nsAString& aURL);
 
   // Add a url + known style sheet to the internal lists:
   nsresult AddNewStyleSheetToList(const nsAString &aURL,
-                                  mozilla::CSSStyleSheet* aStyleSheet);
+                                  mozilla::StyleSheetHandle aStyleSheet);
 
   nsresult RemoveStyleSheetFromList(const nsAString &aURL);
 
@@ -782,7 +782,7 @@ class nsHTMLEditor final : public nsPlaintextEditor,
 
   // Maintain a list of associated style sheets and their urls.
   nsTArray<nsString> mStyleSheetURLs;
-  nsTArray<RefPtr<mozilla::CSSStyleSheet>> mStyleSheets;
+  nsTArray<mozilla::StyleSheetHandle::RefPtr> mStyleSheets;
 
   // an array for holding default style settings
   nsTArray<PropItem*> mDefaultStyles;
diff --git a/editor/libeditor/nsStyleSheetTxns.cpp b/editor/libeditor/nsStyleSheetTxns.cpp
index a7b2325ae8298..bbc4a646ff11d 100644
--- a/editor/libeditor/nsStyleSheetTxns.cpp
+++ b/editor/libeditor/nsStyleSheetTxns.cpp
@@ -9,7 +9,8 @@
 
 #include "nsAString.h"
 #include "nsCOMPtr.h"                   // for nsCOMPtr, do_QueryInterface, etc
-#include "mozilla/CSSStyleSheet.h"      // for mozilla::CSSStyleSheet
+#include "mozilla/StyleSheetHandle.h"   // for mozilla::StyleSheetHandle
+#include "mozilla/StyleSheetHandleInlines.h"
 #include "nsDebug.h"                    // for NS_ENSURE_TRUE
 #include "nsError.h"                    // for NS_OK, etc
 #include "nsIDOMDocument.h"             // for nsIDOMDocument
@@ -20,7 +21,7 @@
 using namespace mozilla;
 
 static void
-AddStyleSheet(nsIEditor* aEditor, CSSStyleSheet* aSheet)
+AddStyleSheet(nsIEditor* aEditor, StyleSheetHandle aSheet)
 {
   nsCOMPtr<nsIDOMDocument> domDoc;
   aEditor->GetDocument(getter_AddRefs(domDoc));
@@ -33,7 +34,7 @@ AddStyleSheet(nsIEditor* aEditor, CSSStyleSheet* aSheet)
 }
 
 static void
-RemoveStyleSheet(nsIEditor* aEditor, CSSStyleSheet* aSheet)
+RemoveStyleSheet(nsIEditor* aEditor, StyleSheetHandle aSheet)
 {
   nsCOMPtr<nsIDOMDocument> domDoc;
   aEditor->GetDocument(getter_AddRefs(domDoc));
@@ -58,7 +59,7 @@ NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(AddStyleSheetTxn)
 NS_INTERFACE_MAP_END_INHERITING(EditTxn)
 
 NS_IMETHODIMP
-AddStyleSheetTxn::Init(nsIEditor *aEditor, CSSStyleSheet *aSheet)
+AddStyleSheetTxn::Init(nsIEditor* aEditor, StyleSheetHandle aSheet)
 {
   NS_ENSURE_TRUE(aEditor && aSheet, NS_ERROR_INVALID_ARG);
 
@@ -108,7 +109,7 @@ NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(RemoveStyleSheetTxn)
 NS_INTERFACE_MAP_END_INHERITING(EditTxn)
 
 NS_IMETHODIMP
-RemoveStyleSheetTxn::Init(nsIEditor *aEditor, CSSStyleSheet *aSheet)
+RemoveStyleSheetTxn::Init(nsIEditor* aEditor, StyleSheetHandle aSheet)
 {
   NS_ENSURE_TRUE(aEditor && aSheet, NS_ERROR_INVALID_ARG);
 
diff --git a/editor/libeditor/nsStyleSheetTxns.h b/editor/libeditor/nsStyleSheetTxns.h
index fe63b3ca75252..281e4d1adf17f 100644
--- a/editor/libeditor/nsStyleSheetTxns.h
+++ b/editor/libeditor/nsStyleSheetTxns.h
@@ -8,10 +8,9 @@
 
 #include "EditTxn.h"                    // for EditTxn, NS_DECL_EDITTXN
 #include "nsAutoPtr.h"                  // for nsRefPtr
-#include "mozilla/CSSStyleSheet.h"      // for mozilla::CSSStyleSheet
+#include "mozilla/StyleSheetHandle.h"   // for mozilla::StyleSheetHandle
 #include "nsCycleCollectionParticipant.h"
 #include "nsID.h"                       // for REFNSIID
-#include "nsISupportsImpl.h"            // for CSSStyleSheet::Release
 #include "nscore.h"                     // for NS_IMETHOD
 
 class nsIEditor;
@@ -23,8 +22,7 @@ class AddStyleSheetTxn : public EditTxn
     * @param aEditor the object providing core editing operations
     * @param aSheet   the stylesheet to add
     */
-  NS_IMETHOD Init(nsIEditor* aEditor,
-                  mozilla::CSSStyleSheet* aSheet);
+  NS_IMETHOD Init(nsIEditor* aEditor, mozilla::StyleSheetHandle aSheet);
 
   AddStyleSheetTxn();
 
@@ -36,7 +34,7 @@ class AddStyleSheetTxn : public EditTxn
 protected:
 
   nsIEditor* mEditor;                      // the editor that created this transaction
-  RefPtr<mozilla::CSSStyleSheet> mSheet; // the style sheet to add
+  mozilla::StyleSheetHandle::RefPtr mSheet; // the style sheet to add
 
 };
 
@@ -48,8 +46,7 @@ class RemoveStyleSheetTxn : public EditTxn
     * @param aEditor the object providing core editing operations
     * @param aSheet   the stylesheet to remove
     */
-  NS_IMETHOD Init(nsIEditor* aEditor,
-                  mozilla::CSSStyleSheet* aSheet);
+  NS_IMETHOD Init(nsIEditor* aEditor, mozilla::StyleSheetHandle aSheet);
 
   RemoveStyleSheetTxn();
 
@@ -61,7 +58,7 @@ class RemoveStyleSheetTxn : public EditTxn
 protected:
 
   nsIEditor* mEditor;                      // the editor that created this transaction
-  RefPtr<mozilla::CSSStyleSheet> mSheet; // the style sheet to remove
+  mozilla::StyleSheetHandle::RefPtr mSheet; // the style sheet to remove
 
 };
 
diff --git a/editor/nsIEditorStyleSheets.idl b/editor/nsIEditorStyleSheets.idl
index 439408ddb16a7..7f8c365e6c32c 100644
--- a/editor/nsIEditorStyleSheets.idl
+++ b/editor/nsIEditorStyleSheets.idl
@@ -5,14 +5,6 @@
 
 #include "nsISupports.idl"
 
-%{C++
-namespace mozilla {
-class CSSStyleSheet;
-} // namespace mozilla
-%}
-
-[ptr] native CSSStyleSheet(mozilla::CSSStyleSheet);
-
 [scriptable, uuid(4805e682-49b9-11d3-9ce4-ed60bd6cb5bc)]
 
 interface nsIEditorStyleSheets : nsISupports
@@ -76,18 +68,4 @@ interface nsIEditorStyleSheets : nsISupports
     * @param aEnable true to enable, or false to disable the style sheet
     */
   void enableStyleSheet(in AString aURL, in boolean aEnable);
-
-  /** Get the CSSStyleSheet associated with the given URL.
-    *
-    * @param aURL         The style sheet's URL
-    * @return             the style sheet
-    */
-  [noscript] CSSStyleSheet getStyleSheetForURL(in AString aURL);
-
-  /** Get the URL associated with the given CSSStyleSheet.
-    *
-    * @param aStyleSheet  The style sheet
-    * @return             the style sheet's URL
-    */
-  [noscript] AString getURLForStyleSheet(in CSSStyleSheet aStyleSheet);
 };
diff --git a/layout/base/nsDocumentViewer.cpp b/layout/base/nsDocumentViewer.cpp
index 6d320868085ef..d8ad6b4b760db 100644
--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -40,6 +40,8 @@
 #include "mozilla/Preferences.h"
 #include "mozilla/dom/EncodingUtils.h"
 #include "mozilla/WeakPtr.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 
 #include "nsViewManager.h"
 #include "nsView.h"
@@ -2170,13 +2172,14 @@ nsDocumentViewer::CreateStyleSet(nsIDocument* aDocument,
     return NS_OK;
   }
 
+  auto cache = nsLayoutStylesheetCache::For(styleSet->BackendType());
+
   // Handle the user sheets.
-  CSSStyleSheet* sheet = nullptr;
+  StyleSheetHandle sheet = nullptr;
   if (nsContentUtils::IsInChromeDocshell(aDocument)) {
-    sheet = nsLayoutStylesheetCache::UserChromeSheet();
-  }
-  else {
-    sheet = nsLayoutStylesheetCache::UserContentSheet();
+    sheet = cache->UserChromeSheet();
+  } else {
+    sheet = cache->UserContentSheet();
   }
 
   if (sheet)
@@ -2189,7 +2192,7 @@ nsDocumentViewer::CreateStyleSet(nsIDocument* aDocument,
   nsCOMPtr<nsIDocShell> ds(mContainer);
   nsCOMPtr<nsIDOMEventTarget> chromeHandler;
   nsCOMPtr<nsIURI> uri;
-  RefPtr<CSSStyleSheet> csssheet;
+  StyleSheetHandle::RefPtr chromeSheet;
 
   if (ds) {
     ds->GetChromeEventHandler(getter_AddRefs(chromeHandler));
@@ -2213,10 +2216,10 @@ nsDocumentViewer::CreateStyleSet(nsIDocument* aDocument,
                     baseURI);
           if (!uri) continue;
 
-          cssLoader->LoadSheetSync(uri, getter_AddRefs(csssheet));
-          if (!csssheet) continue;
+          cssLoader->LoadSheetSync(uri, &chromeSheet);
+          if (!chromeSheet) continue;
 
-          styleSet->PrependStyleSheet(SheetType::Agent, csssheet);
+          styleSet->PrependStyleSheet(SheetType::Agent, chromeSheet);
           shouldOverride = true;
         }
         free(str);
@@ -2225,7 +2228,7 @@ nsDocumentViewer::CreateStyleSet(nsIDocument* aDocument,
   }
 
   if (!shouldOverride) {
-    sheet = nsLayoutStylesheetCache::ScrollbarsSheet();
+    sheet = cache->ScrollbarsSheet();
     if (sheet) {
       styleSet->PrependStyleSheet(SheetType::Agent, sheet);
     }
@@ -2241,12 +2244,12 @@ nsDocumentViewer::CreateStyleSet(nsIDocument* aDocument,
     // an SVG document, and excluding xul.css which will be loaded on demand by
     // nsXULElement::BindToTree.)
 
-    sheet = nsLayoutStylesheetCache::NumberControlSheet();
+    sheet = cache->NumberControlSheet();
     if (sheet) {
       styleSet->PrependStyleSheet(SheetType::Agent, sheet);
     }
 
-    sheet = nsLayoutStylesheetCache::FormsSheet();
+    sheet = cache->FormsSheet();
     if (sheet) {
       styleSet->PrependStyleSheet(SheetType::Agent, sheet);
     }
@@ -2254,33 +2257,33 @@ nsDocumentViewer::CreateStyleSet(nsIDocument* aDocument,
     if (aDocument->LoadsFullXULStyleSheetUpFront()) {
       // nsXULElement::BindToTree loads xul.css on-demand if we don't load it
       // up-front here.
-      sheet = nsLayoutStylesheetCache::XULSheet();
+      sheet = cache->XULSheet();
       if (sheet) {
         styleSet->PrependStyleSheet(SheetType::Agent, sheet);
       }
     }
 
-    sheet = nsLayoutStylesheetCache::MinimalXULSheet();
+    sheet = cache->MinimalXULSheet();
     if (sheet) {
       // Load the minimal XUL rules for scrollbars and a few other XUL things
       // that non-XUL (typically HTML) documents commonly use.
       styleSet->PrependStyleSheet(SheetType::Agent, sheet);
     }
 
-    sheet = nsLayoutStylesheetCache::CounterStylesSheet();
+    sheet = cache->CounterStylesSheet();
     if (sheet) {
       styleSet->PrependStyleSheet(SheetType::Agent, sheet);
     }
 
     if (nsLayoutUtils::ShouldUseNoScriptSheet(aDocument)) {
-      sheet = nsLayoutStylesheetCache::NoScriptSheet();
+      sheet = cache->NoScriptSheet();
       if (sheet) {
         styleSet->PrependStyleSheet(SheetType::Agent, sheet);
       }
     }
 
     if (nsLayoutUtils::ShouldUseNoFramesSheet(aDocument)) {
-      sheet = nsLayoutStylesheetCache::NoFramesSheet();
+      sheet = cache->NoFramesSheet();
       if (sheet) {
         styleSet->PrependStyleSheet(SheetType::Agent, sheet);
       }
@@ -2289,16 +2292,16 @@ nsDocumentViewer::CreateStyleSet(nsIDocument* aDocument,
     // We don't add quirk.css here; nsPresContext::CompatibilityModeChanged will
     // append it if needed.
 
-    sheet = nsLayoutStylesheetCache::HTMLSheet();
+    sheet = cache->HTMLSheet();
     if (sheet) {
       styleSet->PrependStyleSheet(SheetType::Agent, sheet);
     }
 
     styleSet->PrependStyleSheet(SheetType::Agent,
-                                nsLayoutStylesheetCache::UASheet());
+                                cache->UASheet());
   } else {
     // SVG documents may have scrollbars and need the scrollbar styling.
-    sheet = nsLayoutStylesheetCache::MinimalXULSheet();
+    sheet = cache->MinimalXULSheet();
     if (sheet) {
       styleSet->PrependStyleSheet(SheetType::Agent, sheet);
     }
@@ -2306,10 +2309,10 @@ nsDocumentViewer::CreateStyleSet(nsIDocument* aDocument,
 
   nsStyleSheetService* sheetService = nsStyleSheetService::GetInstance();
   if (sheetService) {
-    for (CSSStyleSheet* sheet : *sheetService->AgentStyleSheets()) {
+    for (StyleSheetHandle sheet : *sheetService->AgentStyleSheets()) {
       styleSet->AppendStyleSheet(SheetType::Agent, sheet);
     }
-    for (CSSStyleSheet* sheet : Reversed(*sheetService->UserStyleSheets())) {
+    for (StyleSheetHandle sheet : Reversed(*sheetService->UserStyleSheets())) {
       styleSet->PrependStyleSheet(SheetType::User, sheet);
     }
   }
diff --git a/layout/base/nsIPresShell.h b/layout/base/nsIPresShell.h
index 05a61cf14016c..5df6ac2f05e23 100644
--- a/layout/base/nsIPresShell.h
+++ b/layout/base/nsIPresShell.h
@@ -25,6 +25,7 @@
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/StaticPtr.h"
 #include "mozilla/StyleSetHandle.h"
+#include "mozilla/StyleSheetHandle.h"
 #include "mozilla/WeakPtr.h"
 #include "gfxPoint.h"
 #include "nsTHashtable.h"
@@ -945,23 +946,23 @@ class nsIPresShell : public nsISupports
    * Get the set of agent style sheets for this presentation
    */
   virtual nsresult GetAgentStyleSheets(
-      nsTArray<RefPtr<mozilla::CSSStyleSheet>>& aSheets) = 0;
+      nsTArray<mozilla::StyleSheetHandle::RefPtr>& aSheets) = 0;
 
   /**
    * Replace the set of agent style sheets
    */
   virtual nsresult SetAgentStyleSheets(
-      const nsTArray<RefPtr<mozilla::CSSStyleSheet>>& aSheets) = 0;
+      const nsTArray<mozilla::StyleSheetHandle::RefPtr>& aSheets) = 0;
 
   /**
    * Add an override style sheet for this presentation
    */
-  virtual nsresult AddOverrideStyleSheet(mozilla::CSSStyleSheet* aSheet) = 0;
+  virtual nsresult AddOverrideStyleSheet(mozilla::StyleSheetHandle aSheet) = 0;
 
   /**
    * Remove an override style sheet
    */
-  virtual nsresult RemoveOverrideStyleSheet(mozilla::CSSStyleSheet* aSheet) = 0;
+  virtual nsresult RemoveOverrideStyleSheet(mozilla::StyleSheetHandle aSheet) = 0;
 
   /**
    * Reconstruct frames for all elements in the document
diff --git a/layout/base/nsPresContext.cpp b/layout/base/nsPresContext.cpp
index 8dc3616dc811a..7f2b7a091d490 100644
--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -73,6 +73,8 @@
 #include "gfxTextRun.h"
 #include "nsFontFaceUtils.h"
 #include "nsLayoutStylesheetCache.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 
 #if defined(MOZ_WIDGET_GTK)
 #include "gfxPlatformGtk.h" // xxx - for UseFcFontList
@@ -1357,7 +1359,8 @@ nsPresContext::CompatibilityModeChanged()
   }
 
   StyleSetHandle styleSet = mShell->StyleSet();
-  CSSStyleSheet* sheet = nsLayoutStylesheetCache::QuirkSheet();
+  auto cache = nsLayoutStylesheetCache::For(styleSet->BackendType());
+  StyleSheetHandle sheet = cache->QuirkSheet();
 
   if (needsQuirkSheet) {
     // quirk.css needs to come after html.css; we just keep it at the end.
diff --git a/layout/base/nsPresShell.cpp b/layout/base/nsPresShell.cpp
index ddc2b0d95bb6e..4ddc3f991b815 100644
--- a/layout/base/nsPresShell.cpp
+++ b/layout/base/nsPresShell.cpp
@@ -186,6 +186,8 @@
 #include "mozilla/layers/ScrollInputMethods.h"
 #include "mozilla/StyleSetHandle.h"
 #include "mozilla/StyleSetHandleInlines.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 
 #ifdef ANDROID
 #include "nsIDocShellTreeOwner.h"
@@ -1364,10 +1366,11 @@ PresShell::UpdatePreferenceStyles()
   // matter which pres context we pass in when it does need to be recreated.
   // (See nsPresContext::GetDocumentColorPreferences for how whether we
   // are a chrome origin image affects some pref styling information.)
-  RefPtr<CSSStyleSheet> newPrefSheet =
+  auto cache = nsLayoutStylesheetCache::For(mStyleSet->BackendType());
+  StyleSheetHandle::RefPtr newPrefSheet =
     mPresContext->IsChromeOriginImage() ?
-      nsLayoutStylesheetCache::ChromePreferenceSheet(mPresContext) :
-      nsLayoutStylesheetCache::ContentPreferenceSheet(mPresContext);
+      cache->ChromePreferenceSheet(mPresContext) :
+      cache->ContentPreferenceSheet(mPresContext);
 
   if (mPrefStyleSheet == newPrefSheet) {
     return;
@@ -1406,16 +1409,16 @@ PresShell::AddUserSheet(nsISupports* aSheet)
   mStyleSet->BeginUpdate();
 
   nsStyleSheetService* sheetService = nsStyleSheetService::gInstance;
-  nsTArray<RefPtr<CSSStyleSheet>>& userSheets = *sheetService->UserStyleSheets();
+  nsTArray<StyleSheetHandle::RefPtr>& userSheets = *sheetService->UserStyleSheets();
   // Iterate forwards when removing so the searches for RemoveStyleSheet are as
   // short as possible.
-  for (CSSStyleSheet* sheet : userSheets) {
+  for (StyleSheetHandle sheet : userSheets) {
     mStyleSet->RemoveStyleSheet(SheetType::User, sheet);
   }
 
   // Now iterate backwards, so that the order of userSheets will be the same as
   // the order of sheets from it in the style set.
-  for (CSSStyleSheet* sheet : Reversed(userSheets)) {
+  for (StyleSheetHandle sheet : Reversed(userSheets)) {
     mStyleSet->PrependStyleSheet(SheetType::User, sheet);
   }
 
@@ -1429,8 +1432,10 @@ PresShell::AddAgentSheet(nsISupports* aSheet)
 {
   // Make sure this does what nsDocumentViewer::CreateStyleSet does
   // wrt ordering.
+  // XXXheycam This needs to work with ServoStyleSheets too.
   RefPtr<CSSStyleSheet> sheet = do_QueryObject(aSheet);
   if (!sheet) {
+    NS_ERROR("stylo: AddAgentSheet needs to support ServoStyleSheets");
     return;
   }
 
@@ -1441,14 +1446,16 @@ PresShell::AddAgentSheet(nsISupports* aSheet)
 void
 PresShell::AddAuthorSheet(nsISupports* aSheet)
 {
+  // XXXheycam This needs to work with ServoStyleSheets too.
   RefPtr<CSSStyleSheet> sheet = do_QueryObject(aSheet);
   if (!sheet) {
+    NS_ERROR("stylo: AddAuthorSheet needs to support ServoStyleSheets");
     return;
   }
 
   // Document specific "additional" Author sheets should be stronger than the ones
   // added with the StyleSheetService.
-  CSSStyleSheet* firstAuthorSheet = mDocument->FirstAdditionalAuthorSheet();
+  StyleSheetHandle firstAuthorSheet = mDocument->FirstAdditionalAuthorSheet();
   if (firstAuthorSheet) {
     mStyleSet->InsertStyleSheetBefore(SheetType::Doc, sheet, firstAuthorSheet);
   } else {
@@ -1463,6 +1470,7 @@ PresShell::RemoveSheet(SheetType aType, nsISupports* aSheet)
 {
   RefPtr<CSSStyleSheet> sheet = do_QueryObject(aSheet);
   if (!sheet) {
+    NS_ERROR("stylo: RemoveSheet needs to support ServoStyleSheets");
     return;
   }
 
@@ -4424,7 +4432,7 @@ nsIPresShell::ReconstructStyleDataExternal()
 }
 
 void
-PresShell::RecordStyleSheetChange(CSSStyleSheet* aStyleSheet)
+PresShell::RecordStyleSheetChange(StyleSheetHandle aStyleSheet)
 {
   // too bad we can't check that the update is UPDATE_STYLE
   NS_ASSERTION(mUpdateCount != 0, "must be in an update");
@@ -4432,20 +4440,22 @@ PresShell::RecordStyleSheetChange(CSSStyleSheet* aStyleSheet)
   if (mStylesHaveChanged)
     return;
 
-  RefPtr<CSSStyleSheet> cssStyleSheet = do_QueryObject(aStyleSheet);
-  if (cssStyleSheet) {
-    Element* scopeElement = cssStyleSheet->GetScopeElement();
+  if (aStyleSheet->IsGecko()) {
+    // XXXheycam ServoStyleSheets don't support <style scoped> yet.
+    Element* scopeElement = aStyleSheet->AsGecko()->GetScopeElement();
     if (scopeElement) {
       mChangedScopeStyleRoots.AppendElement(scopeElement);
       return;
     }
+  } else {
+    NS_ERROR("stylo: ServoStyleSheets don't support <style scoped>");
   }
 
   mStylesHaveChanged = true;
 }
 
 void
-PresShell::StyleSheetAdded(CSSStyleSheet* aStyleSheet,
+PresShell::StyleSheetAdded(StyleSheetHandle aStyleSheet,
                            bool aDocumentSheet)
 {
   // We only care when enabled sheets are added
@@ -4457,7 +4467,7 @@ PresShell::StyleSheetAdded(CSSStyleSheet* aStyleSheet,
 }
 
 void
-PresShell::StyleSheetRemoved(CSSStyleSheet* aStyleSheet,
+PresShell::StyleSheetRemoved(StyleSheetHandle aStyleSheet,
                              bool aDocumentSheet)
 {
   // We only care when enabled sheets are removed
@@ -4469,7 +4479,7 @@ PresShell::StyleSheetRemoved(CSSStyleSheet* aStyleSheet,
 }
 
 void
-PresShell::StyleSheetApplicableStateChanged(CSSStyleSheet* aStyleSheet)
+PresShell::StyleSheetApplicableStateChanged(StyleSheetHandle aStyleSheet)
 {
   if (aStyleSheet->HasRules()) {
     RecordStyleSheetChange(aStyleSheet);
@@ -4477,19 +4487,19 @@ PresShell::StyleSheetApplicableStateChanged(CSSStyleSheet* aStyleSheet)
 }
 
 void
-PresShell::StyleRuleChanged(CSSStyleSheet* aStyleSheet)
+PresShell::StyleRuleChanged(StyleSheetHandle aStyleSheet)
 {
   RecordStyleSheetChange(aStyleSheet);
 }
 
 void
-PresShell::StyleRuleAdded(CSSStyleSheet* aStyleSheet)
+PresShell::StyleRuleAdded(StyleSheetHandle aStyleSheet)
 {
   RecordStyleSheetChange(aStyleSheet);
 }
 
 void
-PresShell::StyleRuleRemoved(CSSStyleSheet* aStyleSheet)
+PresShell::StyleRuleRemoved(StyleSheetHandle aStyleSheet)
 {
   RecordStyleSheetChange(aStyleSheet);
 }
@@ -8501,7 +8511,7 @@ PresShell::IsVisible()
 }
 
 nsresult
-PresShell::GetAgentStyleSheets(nsTArray<RefPtr<CSSStyleSheet>>& aSheets)
+PresShell::GetAgentStyleSheets(nsTArray<StyleSheetHandle::RefPtr>& aSheets)
 {
   aSheets.Clear();
   int32_t sheetCount = mStyleSet->SheetCount(SheetType::Agent);
@@ -8511,7 +8521,7 @@ PresShell::GetAgentStyleSheets(nsTArray<RefPtr<CSSStyleSheet>>& aSheets)
   }
 
   for (int32_t i = 0; i < sheetCount; ++i) {
-    CSSStyleSheet* sheet = mStyleSet->StyleSheetAt(SheetType::Agent, i);
+    StyleSheetHandle sheet = mStyleSet->StyleSheetAt(SheetType::Agent, i);
     aSheets.AppendElement(sheet);
   }
 
@@ -8519,19 +8529,19 @@ PresShell::GetAgentStyleSheets(nsTArray<RefPtr<CSSStyleSheet>>& aSheets)
 }
 
 nsresult
-PresShell::SetAgentStyleSheets(const nsTArray<RefPtr<CSSStyleSheet>>& aSheets)
+PresShell::SetAgentStyleSheets(const nsTArray<StyleSheetHandle::RefPtr>& aSheets)
 {
   return mStyleSet->ReplaceSheets(SheetType::Agent, aSheets);
 }
 
 nsresult
-PresShell::AddOverrideStyleSheet(CSSStyleSheet* aSheet)
+PresShell::AddOverrideStyleSheet(StyleSheetHandle aSheet)
 {
   return mStyleSet->PrependStyleSheet(SheetType::Override, aSheet);
 }
 
 nsresult
-PresShell::RemoveOverrideStyleSheet(CSSStyleSheet* aSheet)
+PresShell::RemoveOverrideStyleSheet(StyleSheetHandle aSheet)
 {
   return mStyleSet->RemoveStyleSheet(SheetType::Override, aSheet);
 }
diff --git a/layout/base/nsPresShell.h b/layout/base/nsPresShell.h
index e114cb3aa2536..43565f1503479 100644
--- a/layout/base/nsPresShell.h
+++ b/layout/base/nsPresShell.h
@@ -51,7 +51,6 @@ class nsPresShellEventCB;
 class nsAutoCauseReflowNotifier;
 
 namespace mozilla {
-class CSSStyleSheet;
 class EventDispatchingCallback;
 } // namespace mozilla
 
@@ -155,12 +154,12 @@ class PresShell final : public nsIPresShell,
   virtual void UnsuppressPainting() override;
 
   virtual nsresult GetAgentStyleSheets(
-      nsTArray<RefPtr<mozilla::CSSStyleSheet>>& aSheets) override;
+      nsTArray<mozilla::StyleSheetHandle::RefPtr>& aSheets) override;
   virtual nsresult SetAgentStyleSheets(
-      const nsTArray<RefPtr<mozilla::CSSStyleSheet>>& aSheets) override;
+      const nsTArray<mozilla::StyleSheetHandle::RefPtr>& aSheets) override;
 
-  virtual nsresult AddOverrideStyleSheet(mozilla::CSSStyleSheet* aSheet) override;
-  virtual nsresult RemoveOverrideStyleSheet(mozilla::CSSStyleSheet* aSheet) override;
+  virtual nsresult AddOverrideStyleSheet(mozilla::StyleSheetHandle aSheet) override;
+  virtual nsresult RemoveOverrideStyleSheet(mozilla::StyleSheetHandle aSheet) override;
 
   virtual nsresult HandleEventWithTarget(
                                  mozilla::WidgetEvent* aEvent,
@@ -503,7 +502,7 @@ class PresShell final : public nsIPresShell,
   void ShowEventTargetDebug();
 #endif
 
-  void RecordStyleSheetChange(mozilla::CSSStyleSheet* aStyleSheet);
+  void RecordStyleSheetChange(mozilla::StyleSheetHandle aStyleSheet);
 
   void RemovePreferenceStyles();
 
@@ -781,7 +780,7 @@ class PresShell final : public nsIPresShell,
   nsPoint                   mMouseLocation;
 
   // mStyleSet owns it but we maintain a ref, may be null
-  RefPtr<mozilla::CSSStyleSheet> mPrefStyleSheet;
+  mozilla::StyleSheetHandle::RefPtr mPrefStyleSheet;
 
   // Set of frames that we should mark with NS_FRAME_HAS_DIRTY_CHILDREN after
   // we finish reflowing mCurrentReflowRoot.
diff --git a/layout/base/nsStyleSheetService.cpp b/layout/base/nsStyleSheetService.cpp
index 0817905fd6767..a2218d57bb23c 100644
--- a/layout/base/nsStyleSheetService.cpp
+++ b/layout/base/nsStyleSheetService.cpp
@@ -9,6 +9,8 @@
 #include "nsStyleSheetService.h"
 #include "mozilla/CSSStyleSheet.h"
 #include "mozilla/MemoryReporting.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 #include "mozilla/unused.h"
 #include "mozilla/css/Loader.h"
 #include "mozilla/dom/ContentParent.h"
@@ -78,7 +80,7 @@ nsStyleSheetService::RegisterFromEnumerator(nsICategoryManager  *aManager,
 }
 
 int32_t
-nsStyleSheetService::FindSheetByURI(const nsTArray<RefPtr<CSSStyleSheet>>& aSheets,
+nsStyleSheetService::FindSheetByURI(const nsTArray<StyleSheetHandle::RefPtr>& aSheets,
                                     nsIURI* aSheetURI)
 {
   for (int32_t i = aSheets.Length() - 1; i >= 0; i-- ) {
@@ -152,9 +154,17 @@ nsStyleSheetService::LoadAndRegisterSheet(nsIURI *aSheetURI,
     if (serv) {
       // We're guaranteed that the new sheet is the last sheet in
       // mSheets[aSheetType]
-      CSSStyleSheet* sheet = mSheets[aSheetType].LastElement();
-      serv->NotifyObservers(NS_ISUPPORTS_CAST(nsIDOMCSSStyleSheet*, sheet),
-                            message, nullptr);
+
+      // XXXheycam Once the nsStyleSheetService can hold ServoStyleSheets too,
+      // we'll need to include them in the notification.
+      StyleSheetHandle sheet = mSheets[aSheetType].LastElement();
+      if (sheet->IsGecko()) {
+        CSSStyleSheet* cssSheet = sheet->AsGecko();
+        serv->NotifyObservers(NS_ISUPPORTS_CAST(nsIDOMCSSStyleSheet*, cssSheet),
+                              message, nullptr);
+      } else {
+        NS_ERROR("stylo: can't notify observers of ServoStyleSheets");
+      }
     }
 
     if (XRE_IsParentProcess()) {
@@ -203,9 +213,8 @@ nsStyleSheetService::LoadAndRegisterSheetInternal(nsIURI *aSheetURI,
 
   RefPtr<css::Loader> loader = new css::Loader();
 
-  RefPtr<CSSStyleSheet> sheet;
-  nsresult rv = loader->LoadSheetSync(aSheetURI, parsingMode, true,
-                                      getter_AddRefs(sheet));
+  StyleSheetHandle::RefPtr sheet;
+  nsresult rv = loader->LoadSheetSync(aSheetURI, parsingMode, true, &sheet);
   NS_ENSURE_SUCCESS(rv, rv);
 
   mSheets[aSheetType].AppendElement(sheet);
@@ -253,13 +262,21 @@ nsStyleSheetService::PreloadSheet(nsIURI *aSheetURI, uint32_t aSheetType,
       return NS_ERROR_INVALID_ARG;
   }
 
+  // XXXheycam PreloadSheet can't support ServoStyleSheets until they implement
+  // nsIDOMStyleSheet.
+
   RefPtr<css::Loader> loader = new css::Loader();
 
-  RefPtr<CSSStyleSheet> sheet;
-  nsresult rv = loader->LoadSheetSync(aSheetURI, parsingMode, true,
-                                      getter_AddRefs(sheet));
+  StyleSheetHandle::RefPtr sheet;
+  nsresult rv = loader->LoadSheetSync(aSheetURI, parsingMode, true, &sheet);
   NS_ENSURE_SUCCESS(rv, rv);
-  sheet.forget(aSheet);
+
+  MOZ_ASSERT(sheet->IsGecko(),
+             "stylo: didn't expect Loader to create a ServoStyleSheet");
+
+  RefPtr<CSSStyleSheet> cssSheet = sheet->AsGecko();
+  cssSheet.forget(aSheet);
+
   return NS_OK;
 }
 
@@ -273,7 +290,7 @@ nsStyleSheetService::UnregisterSheet(nsIURI *aSheetURI, uint32_t aSheetType)
 
   int32_t foundIndex = FindSheetByURI(mSheets[aSheetType], aSheetURI);
   NS_ENSURE_TRUE(foundIndex >= 0, NS_ERROR_INVALID_ARG);
-  RefPtr<CSSStyleSheet> sheet = mSheets[aSheetType][foundIndex];
+  StyleSheetHandle::RefPtr sheet = mSheets[aSheetType][foundIndex];
   mSheets[aSheetType].RemoveElementAt(foundIndex);
 
   const char* message;
@@ -291,8 +308,15 @@ nsStyleSheetService::UnregisterSheet(nsIURI *aSheetURI, uint32_t aSheetType)
 
   nsCOMPtr<nsIObserverService> serv = services::GetObserverService();
   if (serv) {
-    serv->NotifyObservers(NS_ISUPPORTS_CAST(nsIDOMCSSStyleSheet*, sheet),
-                          message, nullptr);
+    // XXXheycam Once the nsStyleSheetService can hold ServoStyleSheets too,
+    // we'll need to include them in the notification.
+    if (sheet->IsGecko()) {
+      CSSStyleSheet* cssSheet = sheet->AsGecko();
+      serv->NotifyObservers(NS_ISUPPORTS_CAST(nsIDOMCSSStyleSheet*, cssSheet),
+                            message, nullptr);
+    } else {
+      NS_ERROR("stylo: can't notify observers of ServoStyleSheets");
+    }
   }
 
   if (XRE_IsParentProcess()) {
@@ -347,7 +371,7 @@ nsStyleSheetService::SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) co
   size_t n = aMallocSizeOf(this);
   for (auto& sheetArray : mSheets) {
     n += sheetArray.ShallowSizeOfExcludingThis(aMallocSizeOf);
-    for (CSSStyleSheet* sheet : sheetArray) {
+    for (StyleSheetHandle sheet : sheetArray) {
       n += sheet->SizeOfIncludingThis(aMallocSizeOf);
     }
   }
diff --git a/layout/base/nsStyleSheetService.h b/layout/base/nsStyleSheetService.h
index ec128a6d6caad..81402fcecb7bb 100644
--- a/layout/base/nsStyleSheetService.h
+++ b/layout/base/nsStyleSheetService.h
@@ -15,10 +15,8 @@
 #include "nsIStyleSheetService.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/MemoryReporting.h"
+#include "mozilla/StyleSheetHandle.h"
 
-namespace mozilla {
-class CSSStyleSheet;
-}
 class nsICategoryManager;
 class nsIMemoryReporter;
 class nsISimpleEnumerator;
@@ -43,15 +41,15 @@ class nsStyleSheetService final
 
   nsresult Init();
 
-  nsTArray<RefPtr<mozilla::CSSStyleSheet>>* AgentStyleSheets()
+  nsTArray<mozilla::StyleSheetHandle::RefPtr>* AgentStyleSheets()
   {
     return &mSheets[AGENT_SHEET];
   }
-  nsTArray<RefPtr<mozilla::CSSStyleSheet>>* UserStyleSheets()
+  nsTArray<mozilla::StyleSheetHandle::RefPtr>* UserStyleSheets()
   {
     return &mSheets[USER_SHEET];
   }
-  nsTArray<RefPtr<mozilla::CSSStyleSheet>>* AuthorStyleSheets()
+  nsTArray<mozilla::StyleSheetHandle::RefPtr>* AuthorStyleSheets()
   {
     return &mSheets[AUTHOR_SHEET];
   }
@@ -69,7 +67,7 @@ class nsStyleSheetService final
                                           nsISimpleEnumerator *aEnumerator,
                                           uint32_t             aSheetType);
 
-  int32_t FindSheetByURI(const nsTArray<RefPtr<mozilla::CSSStyleSheet>>& aSheets,
+  int32_t FindSheetByURI(const nsTArray<mozilla::StyleSheetHandle::RefPtr>& aSheets,
                          nsIURI* aSheetURI);
 
   // Like LoadAndRegisterSheet, but doesn't notify.  If successful, the
@@ -77,7 +75,7 @@ class nsStyleSheetService final
   nsresult LoadAndRegisterSheetInternal(nsIURI *aSheetURI,
                                         uint32_t aSheetType);
 
-  nsTArray<RefPtr<mozilla::CSSStyleSheet>> mSheets[3];
+  nsTArray<mozilla::StyleSheetHandle::RefPtr> mSheets[3];
 };
 
 #endif
diff --git a/layout/inspector/inDOMUtils.cpp b/layout/inspector/inDOMUtils.cpp
index f4277789c2014..b1dc77270f137 100644
--- a/layout/inspector/inDOMUtils.cpp
+++ b/layout/inspector/inDOMUtils.cpp
@@ -114,7 +114,9 @@ inDOMUtils::GetAllStyleSheets(nsIDOMDocument *aDocument, uint32_t *aLength,
 
   // Get the document sheets.
   for (int32_t i = 0; i < document->GetNumberOfStyleSheets(); i++) {
-    sheets.AppendElement(document->GetStyleSheetAt(i));
+    // XXXheycam ServoStyleSets don't have the ability to expose their
+    // sheets in a script-accessible way yet.
+    sheets.AppendElement(document->GetStyleSheetAt(i)->AsGecko());
   }
 
   nsISupports** ret = static_cast<nsISupports**>(moz_xmalloc(sheets.Length() *
diff --git a/layout/style/CSSStyleSheet.cpp b/layout/style/CSSStyleSheet.cpp
index 32fb412fa9625..f50f4fb244454 100644
--- a/layout/style/CSSStyleSheet.cpp
+++ b/layout/style/CSSStyleSheet.cpp
@@ -2203,14 +2203,19 @@ CSSStyleSheet::InsertRuleIntoGroup(const nsAString & aRule,
 
 // nsICSSLoaderObserver implementation
 NS_IMETHODIMP
-CSSStyleSheet::StyleSheetLoaded(CSSStyleSheet* aSheet,
+CSSStyleSheet::StyleSheetLoaded(StyleSheetHandle aSheet,
                                 bool aWasAlternate,
                                 nsresult aStatus)
 {
-  if (aSheet->GetParentSheet() == nullptr) {
+  MOZ_ASSERT(aSheet->IsGecko(),
+             "why we were called back with a ServoStyleSheet?");
+
+  CSSStyleSheet* sheet = aSheet->AsGecko();
+
+  if (sheet->GetParentSheet() == nullptr) {
     return NS_OK; // ignore if sheet has been detached already (see parseSheet)
   }
-  NS_ASSERTION(this == aSheet->GetParentSheet(),
+  NS_ASSERTION(this == sheet->GetParentSheet(),
                "We are being notified of a sheet load for a sheet that is not our child!");
 
   if (mDocument && NS_SUCCEEDED(aStatus)) {
@@ -2218,7 +2223,7 @@ CSSStyleSheet::StyleSheetLoaded(CSSStyleSheet* aSheet,
 
     // XXXldb @import rules shouldn't even implement nsIStyleRule (but
     // they do)!
-    mDocument->StyleRuleAdded(this, aSheet->GetOwnerRule());
+    mDocument->StyleRuleAdded(this, sheet->GetOwnerRule());
   }
 
   return NS_OK;
diff --git a/layout/style/CSSStyleSheet.h b/layout/style/CSSStyleSheet.h
index 1a7798ed382cf..d54e0915bf271 100644
--- a/layout/style/CSSStyleSheet.h
+++ b/layout/style/CSSStyleSheet.h
@@ -248,7 +248,7 @@ class CSSStyleSheet final : public nsIDOMCSSStyleSheet,
   nsIURI* GetOriginalURI() const { return mInner->mOriginalSheetURI; }
 
   // nsICSSLoaderObserver interface
-  NS_IMETHOD StyleSheetLoaded(CSSStyleSheet* aSheet, bool aWasAlternate,
+  NS_IMETHOD StyleSheetLoaded(StyleSheetHandle aSheet, bool aWasAlternate,
                               nsresult aStatus) override;
 
   void EnsureUniqueInner();
@@ -282,7 +282,7 @@ class CSSStyleSheet final : public nsIDOMCSSStyleSheet,
   ReferrerPolicy GetReferrerPolicy() const { return mInner->mReferrerPolicy; }
 
   // Get this style sheet's integrity metadata
-  dom::SRIMetadata GetIntegrity() const { return mInner->mIntegrity; }
+  void GetIntegrity(dom::SRIMetadata& aResult) const { aResult = mInner->mIntegrity; }
 
   dom::Element* GetScopeElement() const { return mScopeElement; }
   void SetScopeElement(dom::Element* aScopeElement)
diff --git a/layout/style/FontFaceSet.cpp b/layout/style/FontFaceSet.cpp
index 9d07a8ce27751..83301fd6a8b5b 100644
--- a/layout/style/FontFaceSet.cpp
+++ b/layout/style/FontFaceSet.cpp
@@ -1694,7 +1694,7 @@ FontFaceSet::PrefEnabled()
 // nsICSSLoaderObserver
 
 NS_IMETHODIMP
-FontFaceSet::StyleSheetLoaded(mozilla::CSSStyleSheet* aSheet,
+FontFaceSet::StyleSheetLoaded(StyleSheetHandle aSheet,
                               bool aWasAlternate,
                               nsresult aStatus)
 {
diff --git a/layout/style/FontFaceSet.h b/layout/style/FontFaceSet.h
index 8856d325f0cab..ac452068d0ec1 100644
--- a/layout/style/FontFaceSet.h
+++ b/layout/style/FontFaceSet.h
@@ -144,7 +144,7 @@ class FontFaceSet final : public DOMEventTargetHelper
   static bool PrefEnabled();
 
   // nsICSSLoaderObserver
-  NS_IMETHOD StyleSheetLoaded(mozilla::CSSStyleSheet* aSheet,
+  NS_IMETHOD StyleSheetLoaded(mozilla::StyleSheetHandle aSheet,
                               bool aWasAlternate,
                               nsresult aStatus) override;
 
diff --git a/layout/style/Loader.cpp b/layout/style/Loader.cpp
index 4feb59d251e68..3e36ea103cf6e 100644
--- a/layout/style/Loader.cpp
+++ b/layout/style/Loader.cpp
@@ -51,6 +51,8 @@
 #include "nsINetworkPredictor.h"
 #include "mozilla/dom/ShadowRoot.h"
 #include "mozilla/dom/URL.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 
 #ifdef MOZ_XUL
 #include "nsXULPrototypeCache.h"
@@ -119,7 +121,7 @@ class SheetLoadData final : public nsIRunnable,
   SheetLoadData(Loader* aLoader,
                 const nsSubstring& aTitle,
                 nsIURI* aURI,
-                CSSStyleSheet* aSheet,
+                StyleSheetHandle aSheet,
                 nsIStyleSheetLinkingElement* aOwningElement,
                 bool aIsAlternate,
                 nsICSSLoaderObserver* aObserver,
@@ -129,7 +131,7 @@ class SheetLoadData final : public nsIRunnable,
   // Data for loading a sheet linked from an @import rule
   SheetLoadData(Loader* aLoader,
                 nsIURI* aURI,
-                CSSStyleSheet* aSheet,
+                StyleSheetHandle aSheet,
                 SheetLoadData* aParentData,
                 nsICSSLoaderObserver* aObserver,
                 nsIPrincipal* aLoaderPrincipal,
@@ -138,7 +140,7 @@ class SheetLoadData final : public nsIRunnable,
   // Data for loading a non-document sheet
   SheetLoadData(Loader* aLoader,
                 nsIURI* aURI,
-                CSSStyleSheet* aSheet,
+                StyleSheetHandle aSheet,
                 bool aSyncLoad,
                 SheetParsingMode aParsingMode,
                 bool aUseSystemPrincipal,
@@ -174,7 +176,7 @@ class SheetLoadData final : public nsIRunnable,
   uint32_t                   mLineNumber;
 
   // The sheet we're loading data for
-  RefPtr<CSSStyleSheet>    mSheet;
+  StyleSheetHandle::RefPtr mSheet;
 
   // Linked list of datas for the same URI as us
   SheetLoadData*             mNext;  // strong ref
@@ -305,7 +307,7 @@ NS_IMPL_ISUPPORTS(SheetLoadData, nsIUnicharStreamLoaderObserver, nsIRunnable,
 SheetLoadData::SheetLoadData(Loader* aLoader,
                              const nsSubstring& aTitle,
                              nsIURI* aURI,
-                             CSSStyleSheet* aSheet,
+                             StyleSheetHandle aSheet,
                              nsIStyleSheetLinkingElement* aOwningElement,
                              bool aIsAlternate,
                              nsICSSLoaderObserver* aObserver,
@@ -337,7 +339,7 @@ SheetLoadData::SheetLoadData(Loader* aLoader,
 
 SheetLoadData::SheetLoadData(Loader* aLoader,
                              nsIURI* aURI,
-                             CSSStyleSheet* aSheet,
+                             StyleSheetHandle aSheet,
                              SheetLoadData* aParentData,
                              nsICSSLoaderObserver* aObserver,
                              nsIPrincipal* aLoaderPrincipal,
@@ -378,7 +380,7 @@ SheetLoadData::SheetLoadData(Loader* aLoader,
 
 SheetLoadData::SheetLoadData(Loader* aLoader,
                              nsIURI* aURI,
-                             CSSStyleSheet* aSheet,
+                             StyleSheetHandle aSheet,
                              bool aSyncLoad,
                              SheetParsingMode aParsingMode,
                              bool aUseSystemPrincipal,
@@ -950,7 +952,8 @@ SheetLoadData::OnStreamComplete(nsIUnicharStreamLoader* aLoader,
     }
   }
 
-  SRIMetadata sriMetadata = mSheet->GetIntegrity();
+  SRIMetadata sriMetadata;
+  mSheet->GetIntegrity(sriMetadata);
   if (!sriMetadata.IsEmpty() &&
       NS_FAILED(SRICheck::VerifyIntegrity(sriMetadata, aLoader,
                                           mSheet->GetCORSMode(), aBuffer,
@@ -1068,7 +1071,7 @@ Loader::CreateSheet(nsIURI* aURI,
                     const nsAString& aTitle,
                     StyleSheetState& aSheetState,
                     bool *aIsAlternate,
-                    CSSStyleSheet** aSheet)
+                    StyleSheetHandle::RefPtr* aSheet)
 {
   LOG(("css::Loader::CreateSheet"));
   NS_PRECONDITION(aSheet, "Null out param!");
@@ -1084,9 +1087,10 @@ Loader::CreateSheet(nsIURI* aURI,
   // can mess with our hashtables.
   *aIsAlternate = IsAlternate(aTitle, aHasAlternateRel);
 
+  // XXXheycam Cached sheets currently must be CSSStyleSheets.
   if (aURI) {
     aSheetState = eSheetComplete;
-    RefPtr<CSSStyleSheet> sheet;
+    StyleSheetHandle::RefPtr sheet;
 
     // First, the XUL cache
 #ifdef MOZ_XUL
@@ -1095,7 +1099,7 @@ Loader::CreateSheet(nsIURI* aURI,
       if (cache) {
         if (cache->IsEnabled()) {
           sheet = cache->GetStyleSheet(aURI);
-          LOG(("  From XUL cache: %p", sheet.get()));
+          LOG(("  From XUL cache: %p", sheet->AsVoidPtr()));
         }
       }
     }
@@ -1106,22 +1110,28 @@ Loader::CreateSheet(nsIURI* aURI,
       // Then our per-document complete sheets.
       URIPrincipalReferrerPolicyAndCORSModeHashKey key(aURI, aLoaderPrincipal, aCORSMode, aReferrerPolicy);
 
-      mSheets->mCompleteSheets.Get(&key, getter_AddRefs(sheet));
-      LOG(("  From completed: %p", sheet.get()));
+      StyleSheetHandle completeSheet;
+      mSheets->mCompleteSheets.Get(&key, &completeSheet);
+      sheet = completeSheet;
+      LOG(("  From completed: %p", sheet->AsVoidPtr()));
 
       fromCompleteSheets = !!sheet;
     }
 
     if (sheet) {
+      if (sheet->IsServo()) {
+        MOZ_CRASH("stylo: can't clone ServoStyleSheets yet");
+      }
+
       // This sheet came from the XUL cache or our per-document hashtable; it
       // better be a complete sheet.
-      NS_ASSERTION(sheet->IsComplete(),
+      NS_ASSERTION(sheet->AsGecko()->IsComplete(),
                    "Sheet thinks it's not complete while we think it is");
 
       // Make sure it hasn't been modified; if it has, we can't use it
-      if (sheet->IsModified()) {
+      if (sheet->AsGecko()->IsModified()) {
         LOG(("  Not cloning completed sheet %p because it's been modified",
-             sheet.get()));
+             sheet->AsVoidPtr()));
         sheet = nullptr;
         fromCompleteSheets = false;
       }
@@ -1135,7 +1145,7 @@ Loader::CreateSheet(nsIURI* aURI,
       mSheets->mLoadingDatas.Get(&key, &loadData);
       if (loadData) {
         sheet = loadData->mSheet;
-        LOG(("  From loading: %p", sheet.get()));
+        LOG(("  From loading: %p", sheet->AsVoidPtr()));
 
 #ifdef DEBUG
         bool debugEqual;
@@ -1155,7 +1165,7 @@ Loader::CreateSheet(nsIURI* aURI,
         mSheets->mPendingDatas.Get(&key, &loadData);
         if (loadData) {
           sheet = loadData->mSheet;
-          LOG(("  From pending: %p", sheet.get()));
+          LOG(("  From pending: %p", sheet->AsVoidPtr()));
 
 #ifdef DEBUG
           bool debugEqual;
@@ -1172,20 +1182,28 @@ Loader::CreateSheet(nsIURI* aURI,
 
     if (sheet) {
       // The sheet we have now should be either incomplete or unmodified
-      NS_ASSERTION(!sheet->IsModified() || !sheet->IsComplete(),
+      if (sheet->IsServo()) {
+        MOZ_CRASH("stylo: can't clone ServoStyleSheets yet");
+      }
+      NS_ASSERTION(!sheet->AsGecko()->IsModified() ||
+                   !sheet->AsGecko()->IsComplete(),
                    "Unexpected modified complete sheet");
-      NS_ASSERTION(sheet->IsComplete() || aSheetState != eSheetComplete,
+      NS_ASSERTION(sheet->AsGecko()->IsComplete() ||
+                   aSheetState != eSheetComplete,
                    "Sheet thinks it's not complete while we think it is");
 
-      *aSheet = sheet->Clone(nullptr, nullptr, nullptr, nullptr).take();
+      RefPtr<CSSStyleSheet> clonedSheet =
+        sheet->AsGecko()->Clone(nullptr, nullptr, nullptr, nullptr);
+      *aSheet = Move(clonedSheet);
       if (*aSheet && fromCompleteSheets &&
-          !sheet->GetOwnerNode() && !sheet->GetParentSheet()) {
+          !sheet->AsGecko()->GetOwnerNode() &&
+          !sheet->AsGecko()->GetParentSheet()) {
         // The sheet we're cloning isn't actually referenced by
         // anyone.  Replace it in the cache, so that if our CSSOM is
         // later modified we don't end up with two copies of our inner
         // hanging around.
         URIPrincipalReferrerPolicyAndCORSModeHashKey key(aURI, aLoaderPrincipal, aCORSMode, aReferrerPolicy);
-        NS_ASSERTION((*aSheet)->IsComplete(),
+        NS_ASSERTION((*aSheet)->AsGecko()->IsComplete(),
                      "Should only be caching complete sheets");
         mSheets->mCompleteSheets.Put(&key, *aSheet);
       }
@@ -1218,11 +1236,8 @@ Loader::CreateSheet(nsIURI* aURI,
       SRICheck::IntegrityMetadata(aIntegrity, mDocument, &sriMetadata);
     }
 
-    RefPtr<CSSStyleSheet> sheet = new CSSStyleSheet(aCORSMode,
-                                                      aReferrerPolicy,
-                                                      sriMetadata);
-    sheet->SetURIs(sheetURI, originalURI, baseURI);
-    sheet.forget(aSheet);
+    *aSheet = new CSSStyleSheet(aCORSMode, aReferrerPolicy, sriMetadata);
+    (*aSheet)->SetURIs(sheetURI, originalURI, baseURI);
   }
 
   NS_ASSERTION(*aSheet, "We should have a sheet by now!");
@@ -1238,7 +1253,7 @@ Loader::CreateSheet(nsIURI* aURI,
  * the sheet had "alternate" in its rel.
  */
 void
-Loader::PrepareSheet(CSSStyleSheet* aSheet,
+Loader::PrepareSheet(StyleSheetHandle aSheet,
                      const nsSubstring& aTitle,
                      const nsSubstring& aMediaString,
                      nsMediaList* aMediaList,
@@ -1247,6 +1262,14 @@ Loader::PrepareSheet(CSSStyleSheet* aSheet,
 {
   NS_PRECONDITION(aSheet, "Must have a sheet!");
 
+  // XXXheycam Need to set media, title, etc. on ServoStyleSheets.
+  if (aSheet->IsServo()) {
+    NS_ERROR("stylo: should set metadata on ServoStyleSheets");
+    return;
+  }
+
+  CSSStyleSheet* sheet = aSheet->AsGecko();
+
   RefPtr<nsMediaList> mediaList(aMediaList);
 
   if (!aMediaString.IsEmpty()) {
@@ -1261,11 +1284,11 @@ Loader::PrepareSheet(CSSStyleSheet* aSheet,
     mediumParser.ParseMediaList(aMediaString, nullptr, 0, mediaList, true);
   }
 
-  aSheet->SetMedia(mediaList);
+  sheet->SetMedia(mediaList);
 
-  aSheet->SetTitle(aTitle);
-  aSheet->SetEnabled(! isAlternate);
-  aSheet->SetScopeElement(aScopeElement);
+  sheet->SetTitle(aTitle);
+  sheet->SetEnabled(!isAlternate);
+  sheet->SetScopeElement(aScopeElement);
 }
 
 /**
@@ -1282,7 +1305,7 @@ Loader::PrepareSheet(CSSStyleSheet* aSheet,
  *    as determined by CompareDocumentPosition.
  */
 nsresult
-Loader::InsertSheetInDoc(CSSStyleSheet* aSheet,
+Loader::InsertSheetInDoc(StyleSheetHandle aSheet,
                          nsIContent* aLinkingContent,
                          nsIDocument* aDocument)
 {
@@ -1304,10 +1327,9 @@ Loader::InsertSheetInDoc(CSSStyleSheet* aSheet,
    */
   int32_t insertionPoint;
   for (insertionPoint = sheetCount - 1; insertionPoint >= 0; --insertionPoint) {
-    CSSStyleSheet* curSheet = aDocument->GetStyleSheetAt(insertionPoint);
+    StyleSheetHandle curSheet = aDocument->GetStyleSheetAt(insertionPoint);
     NS_ASSERTION(curSheet, "There must be a sheet here!");
-    nsCOMPtr<nsIDOMNode> sheetOwner;
-    curSheet->GetOwnerNode(getter_AddRefs(sheetOwner));
+    nsCOMPtr<nsINode> sheetOwner = curSheet->GetOwnerNode();
     if (sheetOwner && !aLinkingContent) {
       // Keep moving; all sheets with a sheetOwner come after all
       // sheets without a linkingNode
@@ -1320,12 +1342,11 @@ Loader::InsertSheetInDoc(CSSStyleSheet* aSheet,
       break;
     }
 
-    nsCOMPtr<nsINode> sheetOwnerNode = do_QueryInterface(sheetOwner);
-    NS_ASSERTION(aLinkingContent != sheetOwnerNode,
+    NS_ASSERTION(aLinkingContent != sheetOwner,
                  "Why do we still have our old sheet?");
 
     // Have to compare
-    if (nsContentUtils::PositionIsBefore(sheetOwnerNode, aLinkingContent)) {
+    if (nsContentUtils::PositionIsBefore(sheetOwner, aLinkingContent)) {
       // The current sheet comes before us, and it better be the first
       // such, because now we break
       break;
@@ -1363,8 +1384,8 @@ Loader::InsertSheetInDoc(CSSStyleSheet* aSheet,
  * bug 1220506.)
  */
 nsresult
-Loader::InsertChildSheet(CSSStyleSheet* aSheet,
-                         CSSStyleSheet* aParentSheet,
+Loader::InsertChildSheet(StyleSheetHandle aSheet,
+                         StyleSheetHandle aParentSheet,
                          ImportRule* aParentRule)
 {
   LOG(("css::Loader::InsertChildSheet"));
@@ -1372,12 +1393,18 @@ Loader::InsertChildSheet(CSSStyleSheet* aSheet,
   NS_PRECONDITION(aParentSheet, "Need a parent to insert into");
   NS_PRECONDITION(aParentSheet, "How did we get imported?");
 
+  // XXXheycam The InsertChildSheet API doesn't work with ServoStyleSheets,
+  // since they won't have Gecko ImportRules in them.
+  if (aSheet->IsServo()) {
+    return NS_ERROR_FAILURE;
+  }
+
   // child sheets should always start out enabled, even if they got
   // cloned off of top-level sheets which were disabled
-  aSheet->SetEnabled(true);
+  aSheet->AsGecko()->SetEnabled(true);
 
   aParentSheet->AppendStyleSheet(aSheet);
-  aParentRule->SetSheet(aSheet); // This sets the ownerRule on the sheet
+  aParentRule->SetSheet(aSheet->AsGecko()); // This sets the ownerRule on the sheet
 
   LOG(("  Inserting into parent sheet"));
   //  LOG(("  Inserting into parent sheet at position %d", insertionPoint));
@@ -1418,7 +1445,8 @@ Loader::LoadSheet(SheetLoadData* aLoadData,
     return NS_BINDING_ABORTED;
   }
 
-  SRIMetadata sriMetadata = aLoadData->mSheet->GetIntegrity();
+  SRIMetadata sriMetadata;
+  aLoadData->mSheet->GetIntegrity(sriMetadata);
 
   if (aLoadData->mSyncLoad) {
     LOG(("  Synchronous load"));
@@ -1709,7 +1737,10 @@ Loader::ParseSheet(const nsAString& aInput,
 
   aCompleted = false;
 
-  nsCSSParser parser(this, aLoadData->mSheet);
+  // XXXheycam ServoStyleSheets don't support parsing their contents yet.
+  MOZ_ASSERT(aLoadData->mSheet->IsGecko(),
+             "stylo: can't parse ServoStyleSheets contents yet");
+  nsCSSParser parser(this, aLoadData->mSheet->AsGecko());
 
   // Push our load data on the stack so any kids can pick it up
   mParsingDatas.AppendElement(aLoadData);
@@ -1830,7 +1861,8 @@ Loader::DoSheetComplete(SheetLoadData* aLoadData, nsresult aStatus,
       // If mSheetAlreadyComplete, then the sheet could well be modified between
       // when we posted the async call to SheetComplete and now, since the sheet
       // was page-accessible during that whole time.
-      MOZ_ASSERT(!data->mSheet->IsModified(),
+      MOZ_ASSERT(!(data->mSheet->IsGecko() &&
+                   data->mSheet->AsGecko()->IsModified()),
                  "should not get marked modified during parsing");
       data->mSheet->SetComplete();
       data->ScheduleLoadEventIfNeeded(aStatus);
@@ -1871,39 +1903,43 @@ Loader::DoSheetComplete(SheetLoadData* aLoadData, nsresult aStatus,
     // one of the sheets that will be kept alive by a document or
     // parent sheet anyway, so that if someone then accesses it via
     // CSSOM we won't have extra clones of the inner lying around.
-    data = aLoadData;
-    CSSStyleSheet* sheet = aLoadData->mSheet;
-    while (data) {
-      if (data->mSheet->GetParentSheet() || data->mSheet->GetOwnerNode()) {
-        sheet = data->mSheet;
-        break;
+    if (aLoadData->mSheet->IsGecko()) {
+      data = aLoadData;
+      CSSStyleSheet* sheet = aLoadData->mSheet->AsGecko();
+      while (data) {
+        if (data->mSheet->GetParentSheet() || data->mSheet->GetOwnerNode()) {
+          sheet = data->mSheet->AsGecko();
+          break;
+        }
+        data = data->mNext;
       }
-      data = data->mNext;
-    }
 #ifdef MOZ_XUL
-    if (IsChromeURI(aLoadData->mURI)) {
-      nsXULPrototypeCache* cache = nsXULPrototypeCache::GetInstance();
-      if (cache && cache->IsEnabled()) {
-        if (!cache->GetStyleSheet(aLoadData->mURI)) {
-          LOG(("  Putting sheet in XUL prototype cache"));
-          NS_ASSERTION(sheet->IsComplete(),
-                       "Should only be caching complete sheets");
-          cache->PutStyleSheet(sheet);
+      if (IsChromeURI(aLoadData->mURI)) {
+        nsXULPrototypeCache* cache = nsXULPrototypeCache::GetInstance();
+        if (cache && cache->IsEnabled()) {
+          if (!cache->GetStyleSheet(aLoadData->mURI)) {
+            LOG(("  Putting sheet in XUL prototype cache"));
+            NS_ASSERTION(sheet->IsComplete(),
+                         "Should only be caching complete sheets");
+            cache->PutStyleSheet(sheet);
+          }
         }
       }
-    }
-    else {
+      else {
 #endif
-      URIPrincipalReferrerPolicyAndCORSModeHashKey key(aLoadData->mURI,
-                                         aLoadData->mLoaderPrincipal,
-                                         aLoadData->mSheet->GetCORSMode(),
-                                         aLoadData->mSheet->GetReferrerPolicy());
-      NS_ASSERTION(sheet->IsComplete(),
-                   "Should only be caching complete sheets");
-      mSheets->mCompleteSheets.Put(&key, sheet);
+        URIPrincipalReferrerPolicyAndCORSModeHashKey key(aLoadData->mURI,
+                                           aLoadData->mLoaderPrincipal,
+                                           aLoadData->mSheet->GetCORSMode(),
+                                           aLoadData->mSheet->GetReferrerPolicy());
+        NS_ASSERTION(sheet->IsComplete(),
+                     "Should only be caching complete sheets");
+        mSheets->mCompleteSheets.Put(&key, sheet);
 #ifdef MOZ_XUL
-    }
+      }
 #endif
+    } else {
+      NS_ERROR("stylo: not caching ServoStyleSheet");
+    }
   }
 
   NS_RELEASE(aLoadData);  // this will release parents and siblings and all that
@@ -1939,12 +1975,12 @@ Loader::LoadInlineStyle(nsIContent* aElement,
   // load data or to CreateSheet().  Also, OK to use CORS_NONE for the CORS
   // mode and mDocument's ReferrerPolicy.
   StyleSheetState state;
-  RefPtr<CSSStyleSheet> sheet;
+  StyleSheetHandle::RefPtr sheet;
   nsresult rv = CreateSheet(nullptr, aElement, nullptr, CORS_NONE,
                             mDocument->GetReferrerPolicy(),
                             EmptyString(), // no inline integrity checks
                             false, false, aTitle, state, aIsAlternate,
-                            getter_AddRefs(sheet));
+                            &sheet);
   NS_ENSURE_SUCCESS(rv, rv);
   NS_ASSERTION(state == eSheetNeedsParser,
                "Inline sheets should not be cached");
@@ -2022,11 +2058,11 @@ Loader::LoadStyleLink(nsIContent* aElement,
   NS_ENSURE_SUCCESS(rv, rv);
 
   StyleSheetState state;
-  RefPtr<CSSStyleSheet> sheet;
+  StyleSheetHandle::RefPtr sheet;
   rv = CreateSheet(aURL, aElement, principal, aCORSMode,
                    aReferrerPolicy, aIntegrity, false,
                    aHasAlternateRel, aTitle, state, aIsAlternate,
-                   getter_AddRefs(sheet));
+                   &sheet);
   NS_ENSURE_SUCCESS(rv, rv);
 
   LOG(("  Sheet is alternate: %d", *aIsAlternate));
@@ -2039,8 +2075,7 @@ Loader::LoadStyleLink(nsIContent* aElement,
   nsCOMPtr<nsIStyleSheetLinkingElement> owningElement(do_QueryInterface(aElement));
 
   if (state == eSheetComplete) {
-    LOG(("  Sheet already complete: 0x%p",
-         static_cast<void*>(sheet.get())));
+    LOG(("  Sheet already complete: 0x%p", sheet->AsVoidPtr()));
     if (aObserver || !mObservers.IsEmpty() || owningElement) {
       rv = PostLoadEvent(aURL, sheet, aObserver, *aIsAlternate,
                          owningElement);
@@ -2110,7 +2145,7 @@ HaveAncestorDataWithURI(SheetLoadData *aData, nsIURI *aURI)
 }
 
 nsresult
-Loader::LoadChildSheet(CSSStyleSheet* aParentSheet,
+Loader::LoadChildSheet(StyleSheetHandle aParentSheet,
                        nsIURI* aURL,
                        nsMediaList* aMedia,
                        ImportRule* aParentRule,
@@ -2127,22 +2162,16 @@ Loader::LoadChildSheet(CSSStyleSheet* aParentSheet,
 
   LOG_URI("  Child uri: '%s'", aURL);
 
-  nsCOMPtr<nsIDOMNode> owningNode;
+  nsCOMPtr<nsINode> owningNode;
 
   // check for an owning document: if none, don't bother walking up the parent
   // sheets
   if (aParentSheet->GetOwningDocument()) {
-    nsCOMPtr<nsIDOMStyleSheet> nextParentSheet(aParentSheet);
-    NS_ENSURE_TRUE(nextParentSheet, NS_ERROR_FAILURE); //Not a stylesheet!?
-
-    nsCOMPtr<nsIDOMStyleSheet> topSheet;
-    //traverse our way to the top-most sheet
-    do {
-      topSheet.swap(nextParentSheet);
-      topSheet->GetParentStyleSheet(getter_AddRefs(nextParentSheet));
-    } while (nextParentSheet);
-
-    topSheet->GetOwnerNode(getter_AddRefs(owningNode));
+    StyleSheetHandle topSheet = aParentSheet;
+    while (StyleSheetHandle parent = topSheet->GetParentSheet()) {
+      topSheet = parent;
+    }
+    owningNode = topSheet->GetOwnerNode();
   }
 
   nsISupports* context = owningNode;
@@ -2175,15 +2204,20 @@ Loader::LoadChildSheet(CSSStyleSheet* aParentSheet,
     LOG(("  No parent load; must be CSSOM"));
     // No parent load data, so the sheet will need to be notified when
     // we finish, if it can be, if we do the load asynchronously.
-    observer = aParentSheet;
+    // XXXheycam ServoStyleSheet doesn't implement nsICSSLoaderObserver yet.
+    MOZ_ASSERT(aParentSheet->IsGecko(),
+               "stylo: ServoStyleSheets don't support child sheet loading yet");
+    observer = aParentSheet->AsGecko();
   }
 
   // Now that we know it's safe to load this (passes security check and not a
   // loop) do so.
-  RefPtr<CSSStyleSheet> sheet;
+  StyleSheetHandle::RefPtr sheet;
+  RefPtr<CSSStyleSheet> reusableSheet;
   StyleSheetState state;
-  if (aReusableSheets && aReusableSheets->FindReusableStyleSheet(aURL, sheet)) {
-    aParentRule->SetSheet(sheet);
+  if (aReusableSheets && aReusableSheets->FindReusableStyleSheet(aURL, reusableSheet)) {
+    sheet = reusableSheet;
+    aParentRule->SetSheet(reusableSheet);
     state = eSheetComplete;
   } else {
     bool isAlternate;
@@ -2193,7 +2227,7 @@ Loader::LoadChildSheet(CSSStyleSheet* aParentSheet,
                      aParentSheet->GetReferrerPolicy(),
                      EmptyString(), // integrity is only checked on main sheet
                      parentData ? parentData->mSyncLoad : false,
-                     false, empty, state, &isAlternate, getter_AddRefs(sheet));
+                     false, empty, state, &isAlternate, &sheet);
     NS_ENSURE_SUCCESS(rv, rv);
 
     PrepareSheet(sheet, empty, empty, aMedia, nullptr, isAlternate);
@@ -2232,7 +2266,7 @@ nsresult
 Loader::LoadSheetSync(nsIURI* aURL,
                       SheetParsingMode aParsingMode,
                       bool aUseSystemPrincipal,
-                      CSSStyleSheet** aSheet)
+                      StyleSheetHandle::RefPtr* aSheet)
 {
   LOG(("css::Loader::LoadSheetSync"));
   return InternalLoadNonDocumentSheet(aURL,
@@ -2246,7 +2280,7 @@ Loader::LoadSheet(nsIURI* aURL,
                   nsIPrincipal* aOriginPrincipal,
                   const nsCString& aCharset,
                   nsICSSLoaderObserver* aObserver,
-                  CSSStyleSheet** aSheet)
+                  StyleSheetHandle::RefPtr* aSheet)
 {
   LOG(("css::Loader::LoadSheet(aURL, aObserver, aSheet) api call"));
   NS_PRECONDITION(aSheet, "aSheet is null");
@@ -2281,7 +2315,7 @@ Loader::InternalLoadNonDocumentSheet(nsIURI* aURL,
                                      bool aUseSystemPrincipal,
                                      nsIPrincipal* aOriginPrincipal,
                                      const nsCString& aCharset,
-                                     CSSStyleSheet** aSheet,
+                                     StyleSheetHandle::RefPtr* aSheet,
                                      nsICSSLoaderObserver* aObserver,
                                      CORSMode aCORSMode,
                                      ReferrerPolicy aReferrerPolicy,
@@ -2309,13 +2343,13 @@ Loader::InternalLoadNonDocumentSheet(nsIURI* aURL,
 
   StyleSheetState state;
   bool isAlternate;
-  RefPtr<CSSStyleSheet> sheet;
+  StyleSheetHandle::RefPtr sheet;
   bool syncLoad = (aObserver == nullptr);
   const nsSubstring& empty = EmptyString();
 
   rv = CreateSheet(aURL, nullptr, aOriginPrincipal, aCORSMode,
                    aReferrerPolicy, aIntegrity, syncLoad, false,
-                   empty, state, &isAlternate, getter_AddRefs(sheet));
+                   empty, state, &isAlternate, &sheet);
   NS_ENSURE_SUCCESS(rv, rv);
 
   PrepareSheet(sheet, empty, empty, nullptr, nullptr, isAlternate);
@@ -2352,7 +2386,7 @@ Loader::InternalLoadNonDocumentSheet(nsIURI* aURL,
 
 nsresult
 Loader::PostLoadEvent(nsIURI* aURI,
-                      CSSStyleSheet* aSheet,
+                      StyleSheetHandle aSheet,
                       nsICSSLoaderObserver* aObserver,
                       bool aWasAlternate,
                       nsIStyleSheetLinkingElement* aElement)
@@ -2535,7 +2569,10 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(Loader)
          !iter.Done();
          iter.Next()) {
       NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "Sheet cache nsCSSLoader");
-      cb.NoteXPCOMChild(NS_ISUPPORTS_CAST(nsIDOMCSSStyleSheet*, iter.UserData()));
+      if (iter.UserData()->IsGecko()) {
+        CSSStyleSheet* sheet = iter.UserData()->AsGecko();
+        cb.NoteXPCOMChild(NS_ISUPPORTS_CAST(nsIDOMCSSStyleSheet*, sheet));
+      }
     }
   }
   nsTObserverArray<nsCOMPtr<nsICSSLoaderObserver>>::ForwardIterator
@@ -2569,10 +2606,10 @@ Loader::SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const
       // If aSheet has a parent, then its parent will report it so we don't
       // have to worry about it here. Likewise, if aSheet has an owning node,
       // then the document that node is in will report it.
-      const RefPtr<CSSStyleSheet>& aSheet = iter.Data();
-      n += (aSheet->GetOwnerNode() || aSheet->GetParentSheet())
+      const StyleSheetHandle sheet = iter.UserData();
+      n += (sheet->GetOwnerNode() || sheet->GetParentSheet())
          ? 0
-         : aSheet->SizeOfIncludingThis(aMallocSizeOf);
+         : sheet->SizeOfIncludingThis(aMallocSizeOf);
     }
   }
   n += mObservers.ShallowSizeOfExcludingThis(aMallocSizeOf);
diff --git a/layout/style/Loader.h b/layout/style/Loader.h
index 81554866222f0..81afed2e178f3 100644
--- a/layout/style/Loader.h
+++ b/layout/style/Loader.h
@@ -22,6 +22,7 @@
 #include "mozilla/CORSMode.h"
 #include "mozilla/CSSStyleSheet.h"
 #include "mozilla/MemoryReporting.h"
+#include "mozilla/StyleSheetHandle.h"
 #include "mozilla/net/ReferrerPolicy.h"
 
 class nsICSSLoaderObserver;
@@ -286,7 +287,7 @@ class Loader final {
    * @param aSavedSheets any saved style sheets which could be reused
    *              for this load
    */
-  nsresult LoadChildSheet(CSSStyleSheet* aParentSheet,
+  nsresult LoadChildSheet(StyleSheetHandle aParentSheet,
                           nsIURI* aURL,
                           nsMediaList* aMedia,
                           ImportRule* aRule,
@@ -317,13 +318,13 @@ class Loader final {
   nsresult LoadSheetSync(nsIURI* aURL,
                          SheetParsingMode aParsingMode,
                          bool aUseSystemPrincipal,
-                         CSSStyleSheet** aSheet);
+                         StyleSheetHandle::RefPtr* aSheet);
 
   /**
    * As above, but defaults aParsingMode to eAuthorSheetFeatures and
    * aUseSystemPrincipal to false.
    */
-  nsresult LoadSheetSync(nsIURI* aURL, CSSStyleSheet** aSheet) {
+  nsresult LoadSheetSync(nsIURI* aURL, StyleSheetHandle::RefPtr* aSheet) {
     return LoadSheetSync(aURL, eAuthorSheetFeatures, false, aSheet);
   }
 
@@ -352,7 +353,7 @@ class Loader final {
                      nsIPrincipal* aOriginPrincipal,
                      const nsCString& aCharset,
                      nsICSSLoaderObserver* aObserver,
-                     CSSStyleSheet** aSheet);
+                     StyleSheetHandle::RefPtr* aSheet);
 
   /**
    * Same as above, to be used when the caller doesn't care about the
@@ -461,24 +462,24 @@ class Loader final {
                        const nsAString& aTitle,
                        StyleSheetState& aSheetState,
                        bool *aIsAlternate,
-                       CSSStyleSheet** aSheet);
+                       StyleSheetHandle::RefPtr* aSheet);
 
   // Pass in either a media string or the nsMediaList from the
   // CSSParser.  Don't pass both.
   // This method will set the sheet's enabled state based on isAlternate
-  void PrepareSheet(CSSStyleSheet* aSheet,
+  void PrepareSheet(StyleSheetHandle aSheet,
                     const nsAString& aTitle,
                     const nsAString& aMediaString,
                     nsMediaList* aMediaList,
                     dom::Element* aScopeElement,
                     bool isAlternate);
 
-  nsresult InsertSheetInDoc(CSSStyleSheet* aSheet,
+  nsresult InsertSheetInDoc(StyleSheetHandle aSheet,
                             nsIContent* aLinkingContent,
                             nsIDocument* aDocument);
 
-  nsresult InsertChildSheet(CSSStyleSheet* aSheet,
-                            CSSStyleSheet* aParentSheet,
+  nsresult InsertChildSheet(StyleSheetHandle aSheet,
+                            StyleSheetHandle aParentSheet,
                             ImportRule* aParentRule);
 
   nsresult InternalLoadNonDocumentSheet(nsIURI* aURL,
@@ -487,7 +488,7 @@ class Loader final {
                                         bool aUseSystemPrincipal,
                                         nsIPrincipal* aOriginPrincipal,
                                         const nsCString& aCharset,
-                                        CSSStyleSheet** aSheet,
+                                        StyleSheetHandle::RefPtr* aSheet,
                                         nsICSSLoaderObserver* aObserver,
                                         CORSMode aCORSMode = CORS_NONE,
                                         ReferrerPolicy aReferrerPolicy = mozilla::net::RP_Default,
@@ -501,7 +502,7 @@ class Loader final {
   // sheet was loaded from (may be null for inline sheets).  aElement is the
   // owning element for this sheet.
   nsresult PostLoadEvent(nsIURI* aURI,
-                         CSSStyleSheet* aSheet,
+                         StyleSheetHandle aSheet,
                          nsICSSLoaderObserver* aObserver,
                          bool aWasAlternate,
                          nsIStyleSheetLinkingElement* aElement);
@@ -537,8 +538,9 @@ class Loader final {
                        LoadDataArray& aDatasToNotify);
 
   struct Sheets {
-    nsRefPtrHashtable<URIPrincipalReferrerPolicyAndCORSModeHashKey, CSSStyleSheet>
-                      mCompleteSheets;
+    nsBaseHashtable<URIPrincipalReferrerPolicyAndCORSModeHashKey,
+                    StyleSheetHandle::RefPtr,
+                    StyleSheetHandle> mCompleteSheets;
     nsDataHashtable<URIPrincipalReferrerPolicyAndCORSModeHashKey, SheetLoadData*>
                       mLoadingDatas; // weak refs
     nsDataHashtable<URIPrincipalReferrerPolicyAndCORSModeHashKey, SheetLoadData*>
diff --git a/layout/style/ServoStyleSet.cpp b/layout/style/ServoStyleSet.cpp
index 33bb8d1820cd4..932727d21fa33 100644
--- a/layout/style/ServoStyleSet.cpp
+++ b/layout/style/ServoStyleSet.cpp
@@ -96,36 +96,36 @@ ServoStyleSet::ResolveAnonymousBoxStyle(nsIAtom* aPseudoTag,
 // manage the set of style sheets in the style set
 nsresult
 ServoStyleSet::AppendStyleSheet(SheetType aType,
-                                CSSStyleSheet* aSheet)
+                                ServoStyleSheet* aSheet)
 {
   MOZ_CRASH("stylo: not implemented");
 }
 
 nsresult
 ServoStyleSet::PrependStyleSheet(SheetType aType,
-                                 CSSStyleSheet* aSheet)
+                                 ServoStyleSheet* aSheet)
 {
   MOZ_CRASH("stylo: not implemented");
 }
 
 nsresult
 ServoStyleSet::RemoveStyleSheet(SheetType aType,
-                                CSSStyleSheet* aSheet)
+                                ServoStyleSheet* aSheet)
 {
   MOZ_CRASH("stylo: not implemented");
 }
 
 nsresult
 ServoStyleSet::ReplaceSheets(SheetType aType,
-                             const nsTArray<RefPtr<CSSStyleSheet>>& aNewSheets)
+                             const nsTArray<RefPtr<ServoStyleSheet>>& aNewSheets)
 {
   MOZ_CRASH("stylo: not implemented");
 }
 
 nsresult
 ServoStyleSet::InsertStyleSheetBefore(SheetType aType,
-                                      CSSStyleSheet* aNewSheet,
-                                      CSSStyleSheet* aReferenceSheet)
+                                      ServoStyleSheet* aNewSheet,
+                                      ServoStyleSheet* aReferenceSheet)
 {
   MOZ_CRASH("stylo: not implemented");
 }
@@ -136,7 +136,7 @@ ServoStyleSet::SheetCount(SheetType aType) const
   MOZ_CRASH("stylo: not implemented");
 }
 
-CSSStyleSheet*
+ServoStyleSheet*
 ServoStyleSet::StyleSheetAt(SheetType aType,
                             int32_t aIndex) const
 {
@@ -144,13 +144,13 @@ ServoStyleSet::StyleSheetAt(SheetType aType,
 }
 
 nsresult
-ServoStyleSet::RemoveDocStyleSheet(CSSStyleSheet* aSheet)
+ServoStyleSet::RemoveDocStyleSheet(ServoStyleSheet* aSheet)
 {
   MOZ_CRASH("stylo: not implemented");
 }
 
 nsresult
-ServoStyleSet::AddDocStyleSheet(CSSStyleSheet* aSheet,
+ServoStyleSet::AddDocStyleSheet(ServoStyleSheet* aSheet,
                                 nsIDocument* aDocument)
 {
   MOZ_CRASH("stylo: not implemented");
diff --git a/layout/style/ServoStyleSet.h b/layout/style/ServoStyleSet.h
index 53150f32bd32e..97e40965a1538 100644
--- a/layout/style/ServoStyleSet.h
+++ b/layout/style/ServoStyleSet.h
@@ -20,6 +20,7 @@ namespace dom {
 class Element;
 } // namespace dom
 class CSSStyleSheet;
+class ServoStyleSheet;
 } // namespace mozilla
 class nsStyleContext;
 class nsPresContext;
@@ -69,20 +70,20 @@ class ServoStyleSet
                            uint32_t aFlags = 0);
 
   // manage the set of style sheets in the style set
-  nsresult AppendStyleSheet(SheetType aType, CSSStyleSheet* aSheet);
-  nsresult PrependStyleSheet(SheetType aType, CSSStyleSheet* aSheet);
-  nsresult RemoveStyleSheet(SheetType aType, CSSStyleSheet* aSheet);
+  nsresult AppendStyleSheet(SheetType aType, ServoStyleSheet* aSheet);
+  nsresult PrependStyleSheet(SheetType aType, ServoStyleSheet* aSheet);
+  nsresult RemoveStyleSheet(SheetType aType, ServoStyleSheet* aSheet);
   nsresult ReplaceSheets(SheetType aType,
-                         const nsTArray<RefPtr<CSSStyleSheet>>& aNewSheets);
+                         const nsTArray<RefPtr<ServoStyleSheet>>& aNewSheets);
   nsresult InsertStyleSheetBefore(SheetType aType,
-                                  CSSStyleSheet* aNewSheet,
-                                  CSSStyleSheet* aReferenceSheet);
+                                  ServoStyleSheet* aNewSheet,
+                                  ServoStyleSheet* aReferenceSheet);
 
   int32_t SheetCount(SheetType aType) const;
-  CSSStyleSheet* StyleSheetAt(SheetType aType, int32_t aIndex) const;
+  ServoStyleSheet* StyleSheetAt(SheetType aType, int32_t aIndex) const;
 
-  nsresult RemoveDocStyleSheet(CSSStyleSheet* aSheet);
-  nsresult AddDocStyleSheet(CSSStyleSheet* aSheet, nsIDocument* aDocument);
+  nsresult RemoveDocStyleSheet(ServoStyleSheet* aSheet);
+  nsresult AddDocStyleSheet(ServoStyleSheet* aSheet, nsIDocument* aDocument);
 
   // check whether there is ::before/::after style for an element
   already_AddRefed<nsStyleContext>
diff --git a/layout/style/StyleSetHandle.h b/layout/style/StyleSetHandle.h
index 204216986ad81..20477d2acdf57 100644
--- a/layout/style/StyleSetHandle.h
+++ b/layout/style/StyleSetHandle.h
@@ -11,6 +11,7 @@
 #include "mozilla/RefPtr.h"
 #include "mozilla/SheetType.h"
 #include "mozilla/StyleBackendType.h"
+#include "mozilla/StyleSheetHandle.h"
 #include "nsChangeHint.h"
 #include "nsCSSPseudoElements.h"
 #include "nsTArray.h"
@@ -127,18 +128,18 @@ class StyleSetHandle
     inline already_AddRefed<nsStyleContext>
     ResolveAnonymousBoxStyle(nsIAtom* aPseudoTag, nsStyleContext* aParentContext,
                              uint32_t aFlags = 0);
-    inline nsresult AppendStyleSheet(SheetType aType, CSSStyleSheet* aSheet);
-    inline nsresult PrependStyleSheet(SheetType aType, CSSStyleSheet* aSheet);
-    inline nsresult RemoveStyleSheet(SheetType aType, CSSStyleSheet* aSheet);
+    inline nsresult AppendStyleSheet(SheetType aType, StyleSheetHandle aSheet);
+    inline nsresult PrependStyleSheet(SheetType aType, StyleSheetHandle aSheet);
+    inline nsresult RemoveStyleSheet(SheetType aType, StyleSheetHandle aSheet);
     inline nsresult ReplaceSheets(SheetType aType,
-                           const nsTArray<RefPtr<CSSStyleSheet>>& aNewSheets);
+                           const nsTArray<StyleSheetHandle::RefPtr>& aNewSheets);
     inline nsresult InsertStyleSheetBefore(SheetType aType,
-                                    CSSStyleSheet* aNewSheet,
-                                    CSSStyleSheet* aReferenceSheet);
+                                    StyleSheetHandle aNewSheet,
+                                    StyleSheetHandle aReferenceSheet);
     inline int32_t SheetCount(SheetType aType) const;
-    inline CSSStyleSheet* StyleSheetAt(SheetType aType, int32_t aIndex) const;
-    inline nsresult RemoveDocStyleSheet(CSSStyleSheet* aSheet);
-    inline nsresult AddDocStyleSheet(CSSStyleSheet* aSheet, nsIDocument* aDocument);
+    inline StyleSheetHandle StyleSheetAt(SheetType aType, int32_t aIndex) const;
+    inline nsresult RemoveDocStyleSheet(StyleSheetHandle aSheet);
+    inline nsresult AddDocStyleSheet(StyleSheetHandle aSheet, nsIDocument* aDocument);
     inline already_AddRefed<nsStyleContext>
     ProbePseudoElementStyle(dom::Element* aParentElement,
                             mozilla::CSSPseudoElementType aType,
diff --git a/layout/style/StyleSetHandleInlines.h b/layout/style/StyleSetHandleInlines.h
index 0ac60234295de..f1c8a11c1e1ec 100644
--- a/layout/style/StyleSetHandleInlines.h
+++ b/layout/style/StyleSetHandleInlines.h
@@ -7,11 +7,19 @@
 #ifndef mozilla_StyleSetHandleInlines_h
 #define mozilla_StyleSetHandleInlines_h
 
+#include "mozilla/CSSStyleSheet.h"
 #include "mozilla/ServoStyleSet.h"
+#include "mozilla/ServoStyleSheet.h"
 #include "nsStyleSet.h"
 
-#define FORWARD(method_, args_) \
-  return IsGecko() ? AsGecko()->method_ args_ : AsServo()->method_ args_;
+#define FORWARD_CONCRETE(method_, geckoargs_, servoargs_) \
+  if (IsGecko()) { \
+    return AsGecko()->method_ geckoargs_; \
+  } else { \
+    return AsServo()->method_ servoargs_; \
+  }
+
+#define FORWARD(method_, args_) FORWARD_CONCRETE(method_, args_, args_)
 
 namespace mozilla {
 
@@ -112,36 +120,54 @@ StyleSetHandle::Ptr::ResolveAnonymousBoxStyle(nsIAtom* aPseudoTag,
 
 // manage the set of style sheets in the style set
 nsresult
-StyleSetHandle::Ptr::AppendStyleSheet(SheetType aType, CSSStyleSheet* aSheet)
+StyleSetHandle::Ptr::AppendStyleSheet(SheetType aType, StyleSheetHandle aSheet)
 {
-  FORWARD(AppendStyleSheet, (aType, aSheet));
+  FORWARD_CONCRETE(AppendStyleSheet, (aType, aSheet->AsGecko()),
+                                     (aType, aSheet->AsServo()));
 }
 
 nsresult
-StyleSetHandle::Ptr::PrependStyleSheet(SheetType aType, CSSStyleSheet* aSheet)
+StyleSetHandle::Ptr::PrependStyleSheet(SheetType aType, StyleSheetHandle aSheet)
 {
-  FORWARD(PrependStyleSheet, (aType, aSheet));
+  FORWARD_CONCRETE(PrependStyleSheet, (aType, aSheet->AsGecko()),
+                                      (aType, aSheet->AsServo()));
 }
 
 nsresult
-StyleSetHandle::Ptr::RemoveStyleSheet(SheetType aType, CSSStyleSheet* aSheet)
+StyleSetHandle::Ptr::RemoveStyleSheet(SheetType aType, StyleSheetHandle aSheet)
 {
-  FORWARD(RemoveStyleSheet, (aType, aSheet));
+  FORWARD_CONCRETE(RemoveStyleSheet, (aType, aSheet->AsGecko()),
+                                     (aType, aSheet->AsServo()));
 }
 
 nsresult
 StyleSetHandle::Ptr::ReplaceSheets(SheetType aType,
-                       const nsTArray<RefPtr<CSSStyleSheet>>& aNewSheets)
+                       const nsTArray<StyleSheetHandle::RefPtr>& aNewSheets)
 {
-  FORWARD(ReplaceSheets, (aType, aNewSheets));
+  if (IsGecko()) {
+    nsTArray<RefPtr<CSSStyleSheet>> newSheets(aNewSheets.Length());
+    for (auto& sheet : aNewSheets) {
+      newSheets.AppendElement(sheet->AsGecko());
+    }
+    return AsGecko()->ReplaceSheets(aType, newSheets);
+  } else {
+    nsTArray<RefPtr<ServoStyleSheet>> newSheets(aNewSheets.Length());
+    for (auto& sheet : aNewSheets) {
+      newSheets.AppendElement(sheet->AsServo());
+    }
+    return AsServo()->ReplaceSheets(aType, newSheets);
+  }
 }
 
 nsresult
 StyleSetHandle::Ptr::InsertStyleSheetBefore(SheetType aType,
-                                CSSStyleSheet* aNewSheet,
-                                CSSStyleSheet* aReferenceSheet)
+                                StyleSheetHandle aNewSheet,
+                                StyleSheetHandle aReferenceSheet)
 {
-  FORWARD(InsertStyleSheetBefore, (aType, aNewSheet, aReferenceSheet));
+  FORWARD_CONCRETE(
+    InsertStyleSheetBefore,
+    (aType, aNewSheet->AsGecko(), aReferenceSheet->AsGecko()),
+    (aType, aReferenceSheet->AsServo(), aReferenceSheet->AsServo()));
 }
 
 int32_t
@@ -150,23 +176,25 @@ StyleSetHandle::Ptr::SheetCount(SheetType aType) const
   FORWARD(SheetCount, (aType));
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 StyleSetHandle::Ptr::StyleSheetAt(SheetType aType, int32_t aIndex) const
 {
   FORWARD(StyleSheetAt, (aType, aIndex));
 }
 
 nsresult
-StyleSetHandle::Ptr::RemoveDocStyleSheet(CSSStyleSheet* aSheet)
+StyleSetHandle::Ptr::RemoveDocStyleSheet(StyleSheetHandle aSheet)
 {
-  FORWARD(RemoveDocStyleSheet, (aSheet));
+  FORWARD_CONCRETE(RemoveDocStyleSheet, (aSheet->AsGecko()),
+                                        (aSheet->AsServo()));
 }
 
 nsresult
-StyleSetHandle::Ptr::AddDocStyleSheet(CSSStyleSheet* aSheet,
+StyleSetHandle::Ptr::AddDocStyleSheet(StyleSheetHandle aSheet,
                                       nsIDocument* aDocument)
 {
-  FORWARD(AddDocStyleSheet, (aSheet, aDocument));
+  FORWARD_CONCRETE(AddDocStyleSheet, (aSheet->AsGecko(), aDocument),
+                                     (aSheet->AsServo(), aDocument));
 }
 
 // check whether there is ::before/::after style for an element
diff --git a/layout/style/nsICSSLoaderObserver.h b/layout/style/nsICSSLoaderObserver.h
index db24314ade9e8..79c6b5841f91f 100644
--- a/layout/style/nsICSSLoaderObserver.h
+++ b/layout/style/nsICSSLoaderObserver.h
@@ -9,14 +9,11 @@
 #define nsICSSLoaderObserver_h___
 
 #include "nsISupports.h"
+#include "mozilla/StyleSheetHandle.h"
 
-#define NS_ICSSLOADEROBSERVER_IID     \
-{ 0x7eb90c74, 0xea0c, 0x4df5,       \
-{0xa1, 0x5f, 0x95, 0xf0, 0x6a, 0x98, 0xb9, 0x40} }
-
-namespace mozilla {
-class CSSStyleSheet;
-} // namespace mozilla
+#define NS_ICSSLOADEROBSERVER_IID \
+{ 0xf51fbf2c, 0xfe4b, 0x4a15, \
+  { 0xaf, 0x7e, 0x5e, 0x20, 0x64, 0x5f, 0xaf, 0x58 } }
 
 class nsICSSLoaderObserver : public nsISupports {
 public:
@@ -37,7 +34,7 @@ class nsICSSLoaderObserver : public nsISupports {
    *        as CSS, and doesn't indicate anything about the status of any child
    *        sheets of aSheet.
    */
-  NS_IMETHOD StyleSheetLoaded(mozilla::CSSStyleSheet* aSheet,
+  NS_IMETHOD StyleSheetLoaded(mozilla::StyleSheetHandle aSheet,
                               bool aWasAlternate,
                               nsresult aStatus) = 0;
 };
diff --git a/layout/style/nsLayoutStylesheetCache.cpp b/layout/style/nsLayoutStylesheetCache.cpp
index 85a5f26e45b65..df41e598bc84a 100644
--- a/layout/style/nsLayoutStylesheetCache.cpp
+++ b/layout/style/nsLayoutStylesheetCache.cpp
@@ -10,6 +10,8 @@
 #include "mozilla/CSSStyleSheet.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/Preferences.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 #include "mozilla/css/Loader.h"
 #include "nsIFile.h"
 #include "nsNetUtil.h"
@@ -65,217 +67,185 @@ nsLayoutStylesheetCache::Observe(nsISupports* aSubject,
   return NS_OK;
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::ScrollbarsSheet()
 {
-  EnsureGlobal();
-
-  if (!gStyleCache->mScrollbarsSheet) {
+  if (!mScrollbarsSheet) {
     // Scrollbars don't need access to unsafe rules
     LoadSheetURL("chrome://global/skin/scrollbars.css",
-                 gStyleCache->mScrollbarsSheet, eAuthorSheetFeatures);
+                 mScrollbarsSheet, eAuthorSheetFeatures);
   }
 
-  return gStyleCache->mScrollbarsSheet;
+  return mScrollbarsSheet;
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::FormsSheet()
 {
-  EnsureGlobal();
-
-  if (!gStyleCache->mFormsSheet) {
+  if (!mFormsSheet) {
     // forms.css needs access to unsafe rules
     LoadSheetURL("resource://gre-resources/forms.css",
-                 gStyleCache->mFormsSheet, eAgentSheetFeatures);
+                 mFormsSheet, eAgentSheetFeatures);
   }
 
-  return gStyleCache->mFormsSheet;
+  return mFormsSheet;
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::NumberControlSheet()
 {
-  EnsureGlobal();
-
   if (!sNumberControlEnabled) {
     return nullptr;
   }
 
-  if (!gStyleCache->mNumberControlSheet) {
+  if (!mNumberControlSheet) {
     LoadSheetURL("resource://gre-resources/number-control.css",
-                 gStyleCache->mNumberControlSheet, eAgentSheetFeatures);
+                 mNumberControlSheet, eAgentSheetFeatures);
   }
 
-  return gStyleCache->mNumberControlSheet;
+  return mNumberControlSheet;
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::UserContentSheet()
 {
-  EnsureGlobal();
-  return gStyleCache->mUserContentSheet;
+  return mUserContentSheet;
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::UserChromeSheet()
 {
-  EnsureGlobal();
-  return gStyleCache->mUserChromeSheet;
+  return mUserChromeSheet;
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::UASheet()
 {
-  EnsureGlobal();
-
-  if (!gStyleCache->mUASheet) {
+  if (!mUASheet) {
     LoadSheetURL("resource://gre-resources/ua.css",
-                 gStyleCache->mUASheet, eAgentSheetFeatures);
+                 mUASheet, eAgentSheetFeatures);
   }
 
-  return gStyleCache->mUASheet;
+  return mUASheet;
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::HTMLSheet()
 {
-  EnsureGlobal();
-  return gStyleCache->mHTMLSheet;
+  return mHTMLSheet;
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::MinimalXULSheet()
 {
-  EnsureGlobal();
-  return gStyleCache->mMinimalXULSheet;
+  return mMinimalXULSheet;
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::XULSheet()
 {
-  EnsureGlobal();
-  return gStyleCache->mXULSheet;
+  return mXULSheet;
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::QuirkSheet()
 {
-  EnsureGlobal();
-  return gStyleCache->mQuirkSheet;
+  return mQuirkSheet;
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::SVGSheet()
 {
-  EnsureGlobal();
-  return gStyleCache->mSVGSheet;
+  return mSVGSheet;
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::MathMLSheet()
 {
-  EnsureGlobal();
-
-  if (!gStyleCache->mMathMLSheet) {
+  if (!mMathMLSheet) {
     LoadSheetURL("resource://gre-resources/mathml.css",
-                 gStyleCache->mMathMLSheet, eAgentSheetFeatures);
+                 mMathMLSheet, eAgentSheetFeatures);
   }
 
-  return gStyleCache->mMathMLSheet;
+  return mMathMLSheet;
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::CounterStylesSheet()
 {
-  EnsureGlobal();
-
-  return gStyleCache->mCounterStylesSheet;
+  return mCounterStylesSheet;
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::NoScriptSheet()
 {
-  EnsureGlobal();
-
-  if (!gStyleCache->mNoScriptSheet) {
+  if (!mNoScriptSheet) {
     LoadSheetURL("resource://gre-resources/noscript.css",
-                 gStyleCache->mNoScriptSheet, eAgentSheetFeatures);
+                 mNoScriptSheet, eAgentSheetFeatures);
   }
 
-  return gStyleCache->mNoScriptSheet;
+  return mNoScriptSheet;
 }
 
-CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::NoFramesSheet()
 {
-  EnsureGlobal();
-
-  if (!gStyleCache->mNoFramesSheet) {
+  if (!mNoFramesSheet) {
     LoadSheetURL("resource://gre-resources/noframes.css",
-                 gStyleCache->mNoFramesSheet, eAgentSheetFeatures);
+                 mNoFramesSheet, eAgentSheetFeatures);
   }
 
-  return gStyleCache->mNoFramesSheet;
+  return mNoFramesSheet;
 }
 
-/* static */ CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::ChromePreferenceSheet(nsPresContext* aPresContext)
 {
-  EnsureGlobal();
-
-  if (!gStyleCache->mChromePreferenceSheet) {
-    gStyleCache->BuildPreferenceSheet(gStyleCache->mChromePreferenceSheet,
-                                      aPresContext);
+  if (!mChromePreferenceSheet) {
+    BuildPreferenceSheet(mChromePreferenceSheet, aPresContext);
   }
 
-  return gStyleCache->mChromePreferenceSheet;
+  return mChromePreferenceSheet;
 }
 
-/* static */ CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::ContentPreferenceSheet(nsPresContext* aPresContext)
 {
-  EnsureGlobal();
-
-  if (!gStyleCache->mContentPreferenceSheet) {
-    gStyleCache->BuildPreferenceSheet(gStyleCache->mContentPreferenceSheet,
-                                      aPresContext);
+  if (!mContentPreferenceSheet) {
+    BuildPreferenceSheet(mContentPreferenceSheet, aPresContext);
   }
 
-  return gStyleCache->mContentPreferenceSheet;
+  return mContentPreferenceSheet;
 }
 
-/* static */ CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::ContentEditableSheet()
 {
-  EnsureGlobal();
-
-  if (!gStyleCache->mContentEditableSheet) {
+  if (!mContentEditableSheet) {
     LoadSheetURL("resource://gre/res/contenteditable.css",
-                 gStyleCache->mContentEditableSheet, eAgentSheetFeatures);
+                 mContentEditableSheet, eAgentSheetFeatures);
   }
 
-  return gStyleCache->mContentEditableSheet;
+  return mContentEditableSheet;
 }
 
-/* static */ CSSStyleSheet*
+StyleSheetHandle
 nsLayoutStylesheetCache::DesignModeSheet()
 {
-  EnsureGlobal();
-
-  if (!gStyleCache->mDesignModeSheet) {
+  if (!mDesignModeSheet) {
     LoadSheetURL("resource://gre/res/designmode.css",
-                 gStyleCache->mDesignModeSheet, eAgentSheetFeatures);
+                 mDesignModeSheet, eAgentSheetFeatures);
   }
 
-  return gStyleCache->mDesignModeSheet;
+  return mDesignModeSheet;
 }
 
 void
 nsLayoutStylesheetCache::Shutdown()
 {
   NS_IF_RELEASE(gCSSLoader);
-  gStyleCache = nullptr;
+  gStyleCache_Gecko = nullptr;
+  gStyleCache_Servo = nullptr;
 }
 
 MOZ_DEFINE_MALLOC_SIZE_OF(LayoutStylesheetCacheMallocSizeOf)
@@ -325,7 +295,8 @@ nsLayoutStylesheetCache::SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf
   return n;
 }
 
-nsLayoutStylesheetCache::nsLayoutStylesheetCache()
+nsLayoutStylesheetCache::nsLayoutStylesheetCache(StyleBackendType aType)
+  : mBackendType(aType)
 {
   nsCOMPtr<nsIObserverService> obsSvc =
     mozilla::services::GetObserverService();
@@ -363,7 +334,6 @@ nsLayoutStylesheetCache::nsLayoutStylesheetCache()
 nsLayoutStylesheetCache::~nsLayoutStylesheetCache()
 {
   mozilla::UnregisterWeakMemoryReporter(this);
-  MOZ_ASSERT(!gStyleCache);
 }
 
 void
@@ -372,28 +342,38 @@ nsLayoutStylesheetCache::InitMemoryReporter()
   mozilla::RegisterWeakMemoryReporter(this);
 }
 
-void
-nsLayoutStylesheetCache::EnsureGlobal()
+/* static */ nsLayoutStylesheetCache*
+nsLayoutStylesheetCache::For(StyleBackendType aType)
 {
   MOZ_ASSERT(NS_IsMainThread());
 
-  if (gStyleCache) return;
+  bool mustInit = !gStyleCache_Gecko && !gStyleCache_Servo;
+  auto& cache = aType == StyleBackendType::Gecko ? gStyleCache_Gecko :
+                                                   gStyleCache_Servo;
+
+  if (!cache) {
+    cache = new nsLayoutStylesheetCache(aType);
+    cache->InitMemoryReporter();
+  }
 
-  gStyleCache = new nsLayoutStylesheetCache();
+  if (mustInit) {
+    // Initialization that only needs to be done once for both
+    // nsLayoutStylesheetCaches.
 
-  gStyleCache->InitMemoryReporter();
+    Preferences::AddBoolVarCache(&sNumberControlEnabled, NUMBER_CONTROL_PREF,
+                                 true);
 
-  Preferences::AddBoolVarCache(&sNumberControlEnabled, NUMBER_CONTROL_PREF,
-                               true);
+    // For each pref that controls a CSS feature that a UA style sheet depends
+    // on (such as a pref that enables a property that a UA style sheet uses),
+    // register DependentPrefChanged as a callback to ensure that the relevant
+    // style sheets will be re-parsed.
+    // Preferences::RegisterCallback(&DependentPrefChanged,
+    //                               "layout.css.example-pref.enabled");
+    Preferences::RegisterCallback(&DependentPrefChanged,
+                                  "layout.css.grid.enabled");
+  }
 
-  // For each pref that controls a CSS feature that a UA style sheet depends
-  // on (such as a pref that enables a property that a UA style sheet uses),
-  // register DependentPrefChanged as a callback to ensure that the relevant
-  // style sheets will be re-parsed.
-  // Preferences::RegisterCallback(&DependentPrefChanged,
-  //                               "layout.css.example-pref.enabled");
-  Preferences::RegisterCallback(&DependentPrefChanged,
-                                "layout.css.grid.enabled");
+  return cache;
 }
 
 void
@@ -428,7 +408,7 @@ nsLayoutStylesheetCache::InitFromProfile()
 
 /* static */ void
 nsLayoutStylesheetCache::LoadSheetURL(const char* aURL,
-                                      RefPtr<CSSStyleSheet>& aSheet,
+                                      StyleSheetHandle::RefPtr& aSheet,
                                       SheetParsingMode aParsingMode)
 {
   nsCOMPtr<nsIURI> uri;
@@ -441,7 +421,7 @@ nsLayoutStylesheetCache::LoadSheetURL(const char* aURL,
 
 void
 nsLayoutStylesheetCache::LoadSheetFile(nsIFile* aFile,
-                                       RefPtr<CSSStyleSheet>& aSheet,
+                                       StyleSheetHandle::RefPtr& aSheet,
                                        SheetParsingMode aParsingMode)
 {
   bool exists = false;
@@ -747,7 +727,7 @@ ErrorLoadingBuiltinSheet(nsIURI* aURI, const char* aMsg)
 
 void
 nsLayoutStylesheetCache::LoadSheet(nsIURI* aURI,
-                                   RefPtr<CSSStyleSheet>& aSheet,
+                                   StyleSheetHandle::RefPtr& aSheet,
                                    SheetParsingMode aParsingMode)
 {
   if (!aURI) {
@@ -768,8 +748,7 @@ nsLayoutStylesheetCache::LoadSheet(nsIURI* aURI,
 #ifdef MOZ_CRASHREPORTER
   nsZipArchive::sFileCorruptedReason = nullptr;
 #endif
-  nsresult rv = gCSSLoader->LoadSheetSync(aURI, aParsingMode, true,
-                                          getter_AddRefs(aSheet));
+  nsresult rv = gCSSLoader->LoadSheetSync(aURI, aParsingMode, true, &aSheet);
   if (NS_FAILED(rv)) {
     ErrorLoadingBuiltinSheet(aURI,
       nsPrintfCString("LoadSheetSync failed with error %x", rv).get());
@@ -777,43 +756,78 @@ nsLayoutStylesheetCache::LoadSheet(nsIURI* aURI,
 }
 
 /* static */ void
-nsLayoutStylesheetCache::InvalidateSheet(RefPtr<CSSStyleSheet>& aSheet)
+nsLayoutStylesheetCache::InvalidateSheet(StyleSheetHandle::RefPtr* aGeckoSheet,
+                                         StyleSheetHandle::RefPtr* aServoSheet)
 {
   MOZ_ASSERT(gCSSLoader, "pref changed before we loaded a sheet?");
 
-  if (aSheet) {
-    gCSSLoader->ObsoleteSheet(aSheet->GetSheetURI());
-    aSheet = nullptr;
+  // Make sure sheets have the expected types
+  MOZ_ASSERT(!aGeckoSheet || (*aGeckoSheet)->IsGecko());
+  MOZ_ASSERT(!aServoSheet || (*aServoSheet)->IsServo());
+  // Make sure the URIs match
+  MOZ_ASSERT(!aGeckoSheet || !aServoSheet ||
+             (*aGeckoSheet)->GetSheetURI() == (*aServoSheet)->GetSheetURI(),
+             "Sheets passed should have the same URI");
+
+  nsIURI* uri;
+  if (aGeckoSheet && *aGeckoSheet) {
+    uri = (*aGeckoSheet)->GetSheetURI();
+  } else if (aServoSheet && *aServoSheet) {
+    uri = (*aServoSheet)->GetSheetURI();
+  } else {
+    return;
+  }
+
+  gCSSLoader->ObsoleteSheet(uri);
+  if (aGeckoSheet) {
+    *aGeckoSheet = nullptr;
+  }
+  if (aServoSheet) {
+    *aServoSheet = nullptr;
   }
 }
 
 /* static */ void
 nsLayoutStylesheetCache::DependentPrefChanged(const char* aPref, void* aData)
 {
-  MOZ_ASSERT(gStyleCache, "pref changed after shutdown?");
+  MOZ_ASSERT(gStyleCache_Gecko || gStyleCache_Servo,
+             "pref changed after shutdown?");
 
   // Cause any UA style sheets whose parsing depends on the value of prefs
   // to be re-parsed by dropping the sheet from gCSSLoader's cache then
   // setting our cached sheet pointer to null.  This will only work for sheets
   // that are loaded lazily.
-  InvalidateSheet(gStyleCache->mUASheet); // for layout.css.grid.enabled
+
+#define INVALIDATE(sheet_) \
+  InvalidateSheet(gStyleCache_Gecko ? &gStyleCache_Gecko->sheet_ : nullptr, \
+                  gStyleCache_Servo ? &gStyleCache_Servo->sheet_ : nullptr);
+
+  INVALIDATE(mUASheet);  // for layout.css.grid.enabled
+
+#undef INVALIDATE
 }
 
 /* static */ void
 nsLayoutStylesheetCache::InvalidatePreferenceSheets()
 {
-  if (!gStyleCache) {
-    return;
+  if (gStyleCache_Gecko) {
+    gStyleCache_Gecko->mContentPreferenceSheet = nullptr;
+    gStyleCache_Gecko->mChromePreferenceSheet = nullptr;
+  }
+  if (gStyleCache_Servo) {
+    gStyleCache_Servo->mContentPreferenceSheet = nullptr;
+    gStyleCache_Servo->mChromePreferenceSheet = nullptr;
   }
-
-  gStyleCache->mContentPreferenceSheet = nullptr;
-  gStyleCache->mChromePreferenceSheet = nullptr;
 }
 
 void
-nsLayoutStylesheetCache::BuildPreferenceSheet(RefPtr<CSSStyleSheet>& aSheet,
+nsLayoutStylesheetCache::BuildPreferenceSheet(StyleSheetHandle::RefPtr& aSheet,
                                               nsPresContext* aPresContext)
 {
+  if (mBackendType == StyleBackendType::Servo) {
+    MOZ_CRASH("not implemented");
+  }
+
   aSheet = new CSSStyleSheet(CORS_NONE, mozilla::net::RP_Default);
   aSheet->SetParsingMode(eAgentSheetFeatures);
 
@@ -909,13 +923,16 @@ nsLayoutStylesheetCache::BuildPreferenceSheet(RefPtr<CSSStyleSheet>& aSheet,
                "kPreallocSize should be big enough to build preference style "
                "sheet without reallocation");
 
-  aSheet->ReparseSheet(sheetText);
+  aSheet->AsGecko()->ReparseSheet(sheetText);
 
 #undef NS_GET_R_G_B
 }
 
 mozilla::StaticRefPtr<nsLayoutStylesheetCache>
-nsLayoutStylesheetCache::gStyleCache;
+nsLayoutStylesheetCache::gStyleCache_Gecko;
+
+mozilla::StaticRefPtr<nsLayoutStylesheetCache>
+nsLayoutStylesheetCache::gStyleCache_Servo;
 
 mozilla::css::Loader*
 nsLayoutStylesheetCache::gCSSLoader = nullptr;
diff --git a/layout/style/nsLayoutStylesheetCache.h b/layout/style/nsLayoutStylesheetCache.h
index 16ed6dc2f81f8..5d6c690162264 100644
--- a/layout/style/nsLayoutStylesheetCache.h
+++ b/layout/style/nsLayoutStylesheetCache.h
@@ -13,6 +13,7 @@
 #include "mozilla/Attributes.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/StaticPtr.h"
+#include "mozilla/StyleBackendType.h"
 #include "mozilla/css/Loader.h"
 
 class nsIFile;
@@ -30,27 +31,37 @@ class nsLayoutStylesheetCache final
   NS_DECL_NSIOBSERVER
   NS_DECL_NSIMEMORYREPORTER
 
-  static mozilla::CSSStyleSheet* ScrollbarsSheet();
-  static mozilla::CSSStyleSheet* FormsSheet();
+  /**
+   * Returns the nsLayoutStylesheetCache for the given style backend type.
+   * Callers should pass in a value for aType that matches the style system
+   * backend type for the style set in use.  (A process may call For
+   * and obtain nsLayoutStylesheetCache objects for both backend types,
+   * and a particular UA style sheet might be cached in both, one or neither
+   * nsLayoutStylesheetCache.)
+   */
+  static nsLayoutStylesheetCache* For(mozilla::StyleBackendType aType);
+
+  mozilla::StyleSheetHandle ScrollbarsSheet();
+  mozilla::StyleSheetHandle FormsSheet();
   // This function is expected to return nullptr when the dom.forms.number
   // pref is disabled.
-  static mozilla::CSSStyleSheet* NumberControlSheet();
-  static mozilla::CSSStyleSheet* UserContentSheet();
-  static mozilla::CSSStyleSheet* UserChromeSheet();
-  static mozilla::CSSStyleSheet* UASheet();
-  static mozilla::CSSStyleSheet* HTMLSheet();
-  static mozilla::CSSStyleSheet* MinimalXULSheet();
-  static mozilla::CSSStyleSheet* XULSheet();
-  static mozilla::CSSStyleSheet* QuirkSheet();
-  static mozilla::CSSStyleSheet* SVGSheet();
-  static mozilla::CSSStyleSheet* MathMLSheet();
-  static mozilla::CSSStyleSheet* CounterStylesSheet();
-  static mozilla::CSSStyleSheet* NoScriptSheet();
-  static mozilla::CSSStyleSheet* NoFramesSheet();
-  static mozilla::CSSStyleSheet* ChromePreferenceSheet(nsPresContext* aPresContext);
-  static mozilla::CSSStyleSheet* ContentPreferenceSheet(nsPresContext* aPresContext);
-  static mozilla::CSSStyleSheet* ContentEditableSheet();
-  static mozilla::CSSStyleSheet* DesignModeSheet();
+  mozilla::StyleSheetHandle NumberControlSheet();
+  mozilla::StyleSheetHandle UserContentSheet();
+  mozilla::StyleSheetHandle UserChromeSheet();
+  mozilla::StyleSheetHandle UASheet();
+  mozilla::StyleSheetHandle HTMLSheet();
+  mozilla::StyleSheetHandle MinimalXULSheet();
+  mozilla::StyleSheetHandle XULSheet();
+  mozilla::StyleSheetHandle QuirkSheet();
+  mozilla::StyleSheetHandle SVGSheet();
+  mozilla::StyleSheetHandle MathMLSheet();
+  mozilla::StyleSheetHandle CounterStylesSheet();
+  mozilla::StyleSheetHandle NoScriptSheet();
+  mozilla::StyleSheetHandle NoFramesSheet();
+  mozilla::StyleSheetHandle ChromePreferenceSheet(nsPresContext* aPresContext);
+  mozilla::StyleSheetHandle ContentPreferenceSheet(nsPresContext* aPresContext);
+  mozilla::StyleSheetHandle ContentEditableSheet();
+  mozilla::StyleSheetHandle DesignModeSheet();
 
   static void InvalidatePreferenceSheets();
 
@@ -59,46 +70,48 @@ class nsLayoutStylesheetCache final
   size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
 
 private:
-  nsLayoutStylesheetCache();
+  explicit nsLayoutStylesheetCache(mozilla::StyleBackendType aImpl);
   ~nsLayoutStylesheetCache();
 
-  static void EnsureGlobal();
   void InitFromProfile();
   void InitMemoryReporter();
   static void LoadSheetURL(const char* aURL,
-                           RefPtr<mozilla::CSSStyleSheet>& aSheet,
+                           mozilla::StyleSheetHandle::RefPtr& aSheet,
                            mozilla::css::SheetParsingMode aParsingMode);
   static void LoadSheetFile(nsIFile* aFile,
-                            RefPtr<mozilla::CSSStyleSheet>& aSheet,
+                            mozilla::StyleSheetHandle::RefPtr& aSheet,
                             mozilla::css::SheetParsingMode aParsingMode);
-  static void LoadSheet(nsIURI* aURI, RefPtr<mozilla::CSSStyleSheet>& aSheet,
+  static void LoadSheet(nsIURI* aURI, mozilla::StyleSheetHandle::RefPtr& aSheet,
                         mozilla::css::SheetParsingMode aParsingMode);
-  static void InvalidateSheet(RefPtr<mozilla::CSSStyleSheet>& aSheet);
+  static void InvalidateSheet(mozilla::StyleSheetHandle::RefPtr* aGeckoSheet,
+                              mozilla::StyleSheetHandle::RefPtr* aServoSheet);
   static void DependentPrefChanged(const char* aPref, void* aData);
-  void BuildPreferenceSheet(RefPtr<mozilla::CSSStyleSheet>& aSheet,
+  void BuildPreferenceSheet(mozilla::StyleSheetHandle::RefPtr& aSheet,
                             nsPresContext* aPresContext);
 
-  static mozilla::StaticRefPtr<nsLayoutStylesheetCache> gStyleCache;
+  static mozilla::StaticRefPtr<nsLayoutStylesheetCache> gStyleCache_Gecko;
+  static mozilla::StaticRefPtr<nsLayoutStylesheetCache> gStyleCache_Servo;
   static mozilla::css::Loader* gCSSLoader;
-  RefPtr<mozilla::CSSStyleSheet> mChromePreferenceSheet;
-  RefPtr<mozilla::CSSStyleSheet> mContentEditableSheet;
-  RefPtr<mozilla::CSSStyleSheet> mContentPreferenceSheet;
-  RefPtr<mozilla::CSSStyleSheet> mCounterStylesSheet;
-  RefPtr<mozilla::CSSStyleSheet> mDesignModeSheet;
-  RefPtr<mozilla::CSSStyleSheet> mFormsSheet;
-  RefPtr<mozilla::CSSStyleSheet> mHTMLSheet;
-  RefPtr<mozilla::CSSStyleSheet> mMathMLSheet;
-  RefPtr<mozilla::CSSStyleSheet> mMinimalXULSheet;
-  RefPtr<mozilla::CSSStyleSheet> mNoFramesSheet;
-  RefPtr<mozilla::CSSStyleSheet> mNoScriptSheet;
-  RefPtr<mozilla::CSSStyleSheet> mNumberControlSheet;
-  RefPtr<mozilla::CSSStyleSheet> mQuirkSheet;
-  RefPtr<mozilla::CSSStyleSheet> mSVGSheet;
-  RefPtr<mozilla::CSSStyleSheet> mScrollbarsSheet;
-  RefPtr<mozilla::CSSStyleSheet> mUASheet;
-  RefPtr<mozilla::CSSStyleSheet> mUserChromeSheet;
-  RefPtr<mozilla::CSSStyleSheet> mUserContentSheet;
-  RefPtr<mozilla::CSSStyleSheet> mXULSheet;
+  mozilla::StyleBackendType mBackendType;
+  mozilla::StyleSheetHandle::RefPtr mChromePreferenceSheet;
+  mozilla::StyleSheetHandle::RefPtr mContentEditableSheet;
+  mozilla::StyleSheetHandle::RefPtr mContentPreferenceSheet;
+  mozilla::StyleSheetHandle::RefPtr mCounterStylesSheet;
+  mozilla::StyleSheetHandle::RefPtr mDesignModeSheet;
+  mozilla::StyleSheetHandle::RefPtr mFormsSheet;
+  mozilla::StyleSheetHandle::RefPtr mHTMLSheet;
+  mozilla::StyleSheetHandle::RefPtr mMathMLSheet;
+  mozilla::StyleSheetHandle::RefPtr mMinimalXULSheet;
+  mozilla::StyleSheetHandle::RefPtr mNoFramesSheet;
+  mozilla::StyleSheetHandle::RefPtr mNoScriptSheet;
+  mozilla::StyleSheetHandle::RefPtr mNumberControlSheet;
+  mozilla::StyleSheetHandle::RefPtr mQuirkSheet;
+  mozilla::StyleSheetHandle::RefPtr mSVGSheet;
+  mozilla::StyleSheetHandle::RefPtr mScrollbarsSheet;
+  mozilla::StyleSheetHandle::RefPtr mUASheet;
+  mozilla::StyleSheetHandle::RefPtr mUserChromeSheet;
+  mozilla::StyleSheetHandle::RefPtr mUserContentSheet;
+  mozilla::StyleSheetHandle::RefPtr mXULSheet;
 };
 
 #endif
diff --git a/layout/style/nsMediaFeatures.cpp b/layout/style/nsMediaFeatures.cpp
index 6c2e68c714ee4..7678772443a52 100644
--- a/layout/style/nsMediaFeatures.cpp
+++ b/layout/style/nsMediaFeatures.cpp
@@ -19,6 +19,8 @@
 #include "nsDeviceContext.h"
 #include "nsIDocument.h"
 #include "nsContentUtils.h"
+#include "mozilla/StyleSheetHandle.h"
+#include "mozilla/StyleSheetHandleInlines.h"
 
 using namespace mozilla;
 
diff --git a/layout/style/nsStyleSet.cpp b/layout/style/nsStyleSet.cpp
index f7e706af15556..49877503e03ec 100644
--- a/layout/style/nsStyleSet.cpp
+++ b/layout/style/nsStyleSet.cpp
@@ -745,11 +745,15 @@ nsStyleSet::AddDocStyleSheet(CSSStyleSheet* aSheet, nsIDocument* aDocument)
     if (sheetDocIndex < 0) {
       if (sheetService) {
         auto& authorSheets = *sheetService->AuthorStyleSheets();
-        if (authorSheets.IndexOf(sheet) != authorSheets.NoIndex) {
+        StyleSheetHandle handle = sheet;
+        if (authorSheets.IndexOf(handle) != authorSheets.NoIndex) {
           break;
         }
       }
-      if (sheet == aDocument->FirstAdditionalAuthorSheet()) {
+      MOZ_ASSERT(!aDocument->FirstAdditionalAuthorSheet() ||
+                 aDocument->FirstAdditionalAuthorSheet()->IsGecko(),
+                 "why do we have a ServoStyleSheet for an nsStyleSet?");
+      if (sheet == aDocument->FirstAdditionalAuthorSheet()->GetAsGecko()) {
         break;
       }
     }
@@ -764,6 +768,23 @@ nsStyleSet::AddDocStyleSheet(CSSStyleSheet* aSheet, nsIDocument* aDocument)
   return DirtyRuleProcessors(type);
 }
 
+void
+nsStyleSet::AppendAllXBLStyleSheets(nsTArray<mozilla::CSSStyleSheet*>& aArray) const
+{
+  if (mBindingManager) {
+    // XXXheycam stylo: AppendAllSheets will need to be able to return either
+    // CSSStyleSheets or ServoStyleSheets, on request (and then here requesting
+    // CSSStyleSheets).
+    AutoTArray<StyleSheetHandle, 32> sheets;
+    mBindingManager->AppendAllSheets(sheets);
+    for (StyleSheetHandle handle : sheets) {
+      MOZ_ASSERT(handle->IsGecko(), "stylo: AppendAllSheets shouldn't give us "
+                                    "ServoStyleSheets yet");
+      aArray.AppendElement(handle->AsGecko());
+    }
+  }
+}
+
 nsresult
 nsStyleSet::RemoveDocStyleSheet(CSSStyleSheet* aSheet)
 {
@@ -2497,7 +2518,16 @@ nsStyleSet::EnsureUniqueInnerOnCSSSheets()
   }
 
   if (mBindingManager) {
-    mBindingManager->AppendAllSheets(queue);
+    AutoTArray<StyleSheetHandle, 32> sheets;
+    // XXXheycam stylo: AppendAllSheets will need to be able to return either
+    // CSSStyleSheets or ServoStyleSheets, on request (and then here requesting
+    // CSSStyleSheets).
+    mBindingManager->AppendAllSheets(sheets);
+    for (StyleSheetHandle sheet : sheets) {
+      MOZ_ASSERT(sheet->IsGecko(), "stylo: AppendAllSheets shouldn't give us "
+                                   "ServoStyleSheets yet");
+      queue.AppendElement(sheet->AsGecko());
+    }
   }
 
   while (!queue.IsEmpty()) {
diff --git a/layout/style/nsStyleSet.h b/layout/style/nsStyleSet.h
index 977b7b7939af4..6f0c74f958be3 100644
--- a/layout/style/nsStyleSet.h
+++ b/layout/style/nsStyleSet.h
@@ -338,11 +338,7 @@ class nsStyleSet final
     return mSheets[aType][aIndex];
   }
 
-  void AppendAllXBLStyleSheets(nsTArray<mozilla::CSSStyleSheet*>& aArray) const {
-    if (mBindingManager) {
-      mBindingManager->AppendAllSheets(aArray);
-    }
-  }
+  void AppendAllXBLStyleSheets(nsTArray<mozilla::CSSStyleSheet*>& aArray) const;
 
   nsresult RemoveDocStyleSheet(mozilla::CSSStyleSheet* aSheet);
   nsresult AddDocStyleSheet(mozilla::CSSStyleSheet* aSheet,