diff --git a/dom/base/Document.cpp b/dom/base/Document.cpp
index 5f193f732ac3..6138aec8cb1b 100644
--- a/dom/base/Document.cpp
+++ b/dom/base/Document.cpp
@@ -45,6 +45,7 @@
 #include "nsIObserver.h"
 #include "nsIBaseWindow.h"
 #include "nsILayoutHistoryState.h"
+#include "nsLayoutStylesheetCache.h"
 #include "mozilla/css/Loader.h"
 #include "mozilla/css/ImageLoader.h"
 #include "nsDocShell.h"
@@ -1220,6 +1221,7 @@ Document::Document(const char* aContentType)
       mInXBLUpdate(false),
       mNeedsReleaseAfterStackRefCntRelease(false),
       mStyleSetFilled(false),
+      mQuirkSheetAdded(false),
       mSSApplicableStateNotificationPending(false),
       mMayHaveTitleElement(false),
       mDOMLoadingSet(false),
@@ -1649,10 +1651,6 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INTERNAL(Document)
     cb.NoteXPCOMChild(ToSupports(tmp));
   }
 
-  for (auto iter = tmp->mIdentifierMap.ConstIter(); !iter.Done(); iter.Next()) {
-    iter.Get()->Traverse(&cb);
-  }
-
   tmp->mExternalResourceMap.Traverse(&cb);
 
   // Traverse all Document pointer members.
@@ -1666,7 +1664,6 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INTERNAL(Document)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mParser)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mScriptGlobalObject)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mListenerManager)
-  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mDOMStyleSheets)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mStyleSheetSetList)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mScriptLoader)
 
@@ -1708,7 +1705,6 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INTERNAL(Document)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mPrototypeDocument)
 
   // Traverse all our nsCOMArrays.
-  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mStyleSheets)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mPreloadingImages)
 
   for (uint32_t i = 0; i < tmp->mFrameRequestCallbacks.Length(); ++i) {
@@ -1814,8 +1810,6 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(Document)
     tmp->mListenerManager = nullptr;
   }
 
-  NS_IMPL_CYCLE_COLLECTION_UNLINK(mDOMStyleSheets)
-
   if (tmp->mStyleSheetSetList) {
     tmp->mStyleSheetSetList->Disconnect();
     tmp->mStyleSheetSetList = nullptr;
@@ -1835,7 +1829,6 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(Document)
   // assume that *most* cycles you actually want to break somewhere
   // else, and not unlink an awful lot here.
 
-  tmp->mIdentifierMap.Clear();
   tmp->mExpandoAndGeneration.OwnerUnlinked();
 
   if (tmp->mAnimationController) {
@@ -1913,6 +1906,8 @@ nsresult Document::Init() {
   mFeaturePolicy = new FeaturePolicy(this);
   mFeaturePolicy->SetDefaultOrigin(NodePrincipal());
 
+  mStyleSet = MakeUnique<ServoStyleSet>(*this);
+
   mozilla::HoldJSObjects(this);
 
   return NS_OK;
@@ -2205,15 +2200,12 @@ already_AddRefed<nsIPrincipal> Document::MaybeDowngradePrincipal(
 }
 
 void Document::RemoveDocStyleSheetsFromStyleSets() {
+  MOZ_ASSERT(mStyleSetFilled);
   // The stylesheets should forget us
   for (StyleSheet* sheet : Reversed(mStyleSheets)) {
     sheet->ClearAssociatedDocumentOrShadowRoot();
-
     if (sheet->IsApplicable()) {
-      RefPtr<PresShell> presShell = GetPresShell();
-      if (presShell) {
-        presShell->StyleSet()->RemoveDocStyleSheet(sheet);
-      }
+      mStyleSet->RemoveDocStyleSheet(sheet);
     }
     // XXX Tell observers?
   }
@@ -2224,12 +2216,8 @@ void Document::RemoveStyleSheetsFromStyleSets(
   // The stylesheets should forget us
   for (StyleSheet* sheet : Reversed(aSheets)) {
     sheet->ClearAssociatedDocumentOrShadowRoot();
-
-    if (sheet->IsApplicable()) {
-      RefPtr<PresShell> presShell = GetPresShell();
-      if (presShell) {
-        presShell->StyleSet()->RemoveStyleSheet(aType, sheet);
-      }
+    if (mStyleSetFilled && sheet->IsApplicable()) {
+      mStyleSet->RemoveStyleSheet(aType, sheet);
     }
     // XXX Tell observers?
   }
@@ -2255,8 +2243,6 @@ void Document::ResetStylesheetsToURI(nsIURI* aURI) {
       RemoveStyleSheetsFromStyleSets(*sheetService->AuthorStyleSheets(),
                                      SheetType::Doc);
     }
-
-    mStyleSetFilled = false;
   }
 
   // Release all the sheets
@@ -2281,11 +2267,13 @@ void Document::ResetStylesheetsToURI(nsIURI* aURI) {
     mStyleAttrStyleSheet = new nsHTMLCSSStyleSheet();
   }
 
-  // Now set up our style sets
-  if (PresShell* presShell = GetPresShell()) {
-    FillStyleSet(presShell->StyleSet());
-    if (presShell->StyleSet()->StyleSheetsHaveChanged()) {
-      presShell->ApplicableStylesChanged();
+  if (mStyleSetFilled) {
+    FillStyleSetDocumentSheets();
+
+    if (mStyleSet->StyleSheetsHaveChanged()) {
+      if (PresShell* presShell = GetPresShell()) {
+        presShell->ApplicableStylesChanged();
+      }
     }
   }
 }
@@ -2298,34 +2286,126 @@ static void AppendSheetsToStyleSet(ServoStyleSet* aStyleSet,
   }
 }
 
-void Document::FillStyleSet(ServoStyleSet* aStyleSet) {
-  MOZ_ASSERT(aStyleSet, "Must have a style set");
-  MOZ_ASSERT(aStyleSet->SheetCount(SheetType::Doc) == 0,
-             "Style set already has document sheets?");
+void Document::FillStyleSetUserAndUASheets() {
+  // Make sure this does the same thing as PresShell::Add{User,Agent}Sheet wrt
+  // ordering.
 
+  // The document will fill in the document sheets when we create the presshell
+  auto cache = nsLayoutStylesheetCache::Singleton();
+
+  nsStyleSheetService* sheetService = nsStyleSheetService::GetInstance();
+  MOZ_ASSERT(sheetService,
+             "should never be creating a StyleSet after the style sheet "
+             "service has gone");
+
+  for (StyleSheet* sheet : *sheetService->UserStyleSheets()) {
+    mStyleSet->AppendStyleSheet(SheetType::User, sheet);
+  }
+
+  StyleSheet* sheet = IsInChromeDocShell() ? cache->GetUserChromeSheet()
+                                           : cache->GetUserContentSheet();
+  if (sheet) {
+    mStyleSet->AppendStyleSheet(SheetType::User, sheet);
+  }
+
+  mStyleSet->AppendStyleSheet(SheetType::Agent, cache->UASheet());
+
+  if (MOZ_LIKELY(NodeInfoManager()->MathMLEnabled())) {
+    mStyleSet->AppendStyleSheet(SheetType::Agent, cache->MathMLSheet());
+  }
+
+  if (MOZ_LIKELY(NodeInfoManager()->SVGEnabled())) {
+    mStyleSet->AppendStyleSheet(SheetType::Agent, cache->SVGSheet());
+  }
+
+  mStyleSet->AppendStyleSheet(SheetType::Agent, cache->HTMLSheet());
+
+  if (nsLayoutUtils::ShouldUseNoFramesSheet(this)) {
+    mStyleSet->AppendStyleSheet(SheetType::Agent, cache->NoFramesSheet());
+  }
+
+  if (nsLayoutUtils::ShouldUseNoScriptSheet(this)) {
+    mStyleSet->AppendStyleSheet(SheetType::Agent, cache->NoScriptSheet());
+  }
+
+  mStyleSet->AppendStyleSheet(SheetType::Agent, cache->CounterStylesSheet());
+
+  // Load the minimal XUL rules for scrollbars and a few other XUL things
+  // that non-XUL (typically HTML) documents commonly use.
+  mStyleSet->AppendStyleSheet(SheetType::Agent, cache->MinimalXULSheet());
+
+  // Only load the full XUL sheet if we'll need it.
+  if (LoadsFullXULStyleSheetUpFront()) {
+    mStyleSet->AppendStyleSheet(SheetType::Agent, cache->XULSheet());
+  }
+
+  MOZ_ASSERT(!mQuirkSheetAdded);
+  if (mCompatMode == eCompatibility_NavQuirks) {
+    mStyleSet->AppendStyleSheet(SheetType::Agent, cache->QuirkSheet());
+    mQuirkSheetAdded = true;
+  }
+
+  mStyleSet->AppendStyleSheet(SheetType::Agent, cache->FormsSheet());
+  mStyleSet->AppendStyleSheet(SheetType::Agent, cache->ScrollbarsSheet());
+  mStyleSet->AppendStyleSheet(SheetType::Agent, cache->PluginProblemSheet());
+
+  for (StyleSheet* sheet : *sheetService->AgentStyleSheets()) {
+    mStyleSet->AppendStyleSheet(SheetType::Agent, sheet);
+  }
+}
+
+void Document::FillStyleSet() {
   MOZ_ASSERT(!mStyleSetFilled);
+  FillStyleSetUserAndUASheets();
+  FillStyleSetDocumentSheets();
+  mStyleSetFilled = true;
+}
+
+void Document::FillStyleSetDocumentSheets() {
+  MOZ_ASSERT(mStyleSet->SheetCount(SheetType::Doc) == 0,
+             "Style set already has document sheets?");
 
   for (StyleSheet* sheet : Reversed(mStyleSheets)) {
     if (sheet->IsApplicable()) {
-      aStyleSet->AddDocStyleSheet(sheet, this);
+      mStyleSet->AddDocStyleSheet(sheet, this);
     }
   }
 
-  if (nsStyleSheetService* sheetService = nsStyleSheetService::GetInstance()) {
-    nsTArray<RefPtr<StyleSheet>>& sheets = *sheetService->AuthorStyleSheets();
-    for (StyleSheet* sheet : sheets) {
-      aStyleSet->AppendStyleSheet(SheetType::Doc, sheet);
-    }
+  nsStyleSheetService* sheetService = nsStyleSheetService::GetInstance();
+  for (StyleSheet* sheet : *sheetService->AuthorStyleSheets()) {
+    mStyleSet->AppendStyleSheet(SheetType::Doc, sheet);
   }
 
-  AppendSheetsToStyleSet(aStyleSet, mAdditionalSheets[eAgentSheet],
+  AppendSheetsToStyleSet(mStyleSet.get(), mAdditionalSheets[eAgentSheet],
                          SheetType::Agent);
-  AppendSheetsToStyleSet(aStyleSet, mAdditionalSheets[eUserSheet],
+  AppendSheetsToStyleSet(mStyleSet.get(), mAdditionalSheets[eUserSheet],
                          SheetType::User);
-  AppendSheetsToStyleSet(aStyleSet, mAdditionalSheets[eAuthorSheet],
+  AppendSheetsToStyleSet(mStyleSet.get(), mAdditionalSheets[eAuthorSheet],
                          SheetType::Doc);
+}
 
-  mStyleSetFilled = true;
+void Document::CompatibilityModeChanged() {
+  MOZ_ASSERT(IsHTMLOrXHTML());
+  CSSLoader()->SetCompatibilityMode(mCompatMode);
+  mStyleSet->CompatibilityModeChanged();
+  if (!mStyleSetFilled) {
+    MOZ_ASSERT(!mQuirkSheetAdded);
+    return;
+  }
+  if (mQuirkSheetAdded == NeedsQuirksSheet()) {
+    return;
+  }
+  auto cache = nsLayoutStylesheetCache::Singleton();
+  StyleSheet* sheet = cache->QuirkSheet();
+  if (mQuirkSheetAdded) {
+    mStyleSet->RemoveStyleSheet(SheetType::Agent, sheet);
+  } else {
+    mStyleSet->AppendStyleSheet(SheetType::Agent, sheet);
+  }
+  mQuirkSheetAdded = !mQuirkSheetAdded;
+  if (PresShell* presShell = GetPresShell()) {
+    presShell->ApplicableStylesChanged();
+  }
 }
 
 static void WarnIfSandboxIneffective(nsIDocShell* aDocShell,
@@ -3590,19 +3670,29 @@ static inline void AssertNoStaleServoDataIn(nsINode& aSubtreeRoot) {
 }
 
 already_AddRefed<PresShell> Document::CreatePresShell(
-    nsPresContext* aContext, nsViewManager* aViewManager,
-    UniquePtr<ServoStyleSet> aStyleSet) {
-  NS_ASSERTION(!mPresShell, "We have a presshell already!");
+    nsPresContext* aContext, nsViewManager* aViewManager) {
+  MOZ_ASSERT(!mPresShell, "We have a presshell already!");
 
   NS_ENSURE_FALSE(GetBFCacheEntry(), nullptr);
 
-  FillStyleSet(aStyleSet.get());
   AssertNoStaleServoDataIn(*this);
 
   RefPtr<PresShell> presShell = new PresShell;
   // Note: we don't hold a ref to the shell (it holds a ref to us)
   mPresShell = presShell;
-  presShell->Init(this, aContext, aViewManager, std::move(aStyleSet));
+
+  bool hadStyleSheets = mStyleSetFilled;
+  if (!hadStyleSheets) {
+    FillStyleSet();
+  }
+
+  presShell->Init(this, aContext, aViewManager);
+
+  if (hadStyleSheets) {
+    // Gaining a shell causes changes in how media queries are evaluated, so
+    // invalidate that.
+    aContext->MediaFeatureValuesChanged({MediaFeatureChange::kAllChanges});
+  }
 
   // Make sure to never paint if we belong to an invisible DocShell.
   nsCOMPtr<nsIDocShell> docShell(mDocumentContainer);
@@ -3705,6 +3795,8 @@ void Document::DeletePresShell() {
     presContext->RefreshDriver()->CancelPendingFullscreenEvents(this);
   }
 
+  mStyleSet->ShellDetachedFromDocument();
+
   // When our shell goes away, request that all our images be immediately
   // discarded, so we don't carry around decoded image data for a document we
   // no longer intend to paint.
@@ -3719,7 +3811,6 @@ void Document::DeletePresShell() {
   PresShell* oldPresShell = mPresShell;
   mPresShell = nullptr;
   UpdateFrameRequestCallbackSchedulingState(oldPresShell);
-  mStyleSetFilled = false;
 
   ClearStaleServoData();
   AssertNoStaleServoDataIn(*this);
@@ -3913,9 +4004,11 @@ void Document::RemoveChildNode(nsIContent* aKid, bool aNotify) {
 }
 
 void Document::AddStyleSheetToStyleSets(StyleSheet* aSheet) {
-  if (PresShell* presShell = GetPresShell()) {
-    presShell->StyleSet()->AddDocStyleSheet(aSheet, this);
-    presShell->ApplicableStylesChanged();
+  if (mStyleSetFilled) {
+    mStyleSet->AddDocStyleSheet(aSheet, this);
+    if (PresShell* presShell = GetPresShell()) {
+      presShell->ApplicableStylesChanged();
+    }
   }
 }
 
@@ -3953,9 +4046,11 @@ void Document::NotifyStyleSheetRemoved(StyleSheet* aSheet,
 }
 
 void Document::RemoveStyleSheetFromStyleSets(StyleSheet* aSheet) {
-  if (PresShell* presShell = GetPresShell()) {
-    presShell->StyleSet()->RemoveDocStyleSheet(aSheet);
-    presShell->ApplicableStylesChanged();
+  if (mStyleSetFilled) {
+    mStyleSet->RemoveDocStyleSheet(aSheet);
+    if (PresShell* presShell = GetPresShell()) {
+      presShell->ApplicableStylesChanged();
+    }
   }
 }
 
@@ -4135,10 +4230,12 @@ nsresult Document::AddAdditionalStyleSheet(additionalSheetType aType,
 
   mAdditionalSheets[aType].AppendElement(aSheet);
 
-  if (PresShell* presShell = GetPresShell()) {
+  if (mStyleSetFilled) {
     SheetType type = ConvertAdditionalSheetType(aType);
-    presShell->StyleSet()->AppendStyleSheet(type, aSheet);
-    presShell->ApplicableStylesChanged();
+    mStyleSet->AppendStyleSheet(type, aSheet);
+    if (PresShell* presShell = GetPresShell()) {
+      presShell->ApplicableStylesChanged();
+    }
   }
 
   // Passing false, so documet.styleSheets.length will not be affected by
@@ -4160,10 +4257,12 @@ void Document::RemoveAdditionalStyleSheet(additionalSheetType aType,
 
     if (!mIsGoingAway) {
       MOZ_ASSERT(sheetRef->IsApplicable());
-      if (PresShell* presShell = GetPresShell()) {
+      if (mStyleSetFilled) {
         SheetType type = ConvertAdditionalSheetType(aType);
-        presShell->StyleSet()->RemoveStyleSheet(type, sheetRef);
-        presShell->ApplicableStylesChanged();
+        mStyleSet->RemoveStyleSheet(type, sheetRef);
+        if (PresShell* presShell = GetPresShell()) {
+          presShell->ApplicableStylesChanged();
+        }
       }
     }
 
@@ -11668,8 +11767,7 @@ void Document::FlushUserFontSet() {
 
   if (gfxPlatform::GetPlatform()->DownloadableFontsEnabled()) {
     nsTArray<nsFontFaceRuleContainer> rules;
-    PresShell* presShell = GetPresShell();
-    if (presShell && !presShell->StyleSet()->AppendFontFaceRules(rules)) {
+    if (mStyleSetFilled && !mStyleSet->AppendFontFaceRules(rules)) {
       return;
     }
 
@@ -11687,6 +11785,7 @@ void Document::FlushUserFontSet() {
     // reflect that we're modifying @font-face rules.  (However,
     // without a reflow, nothing will happen to start any downloads
     // that are needed.)
+    PresShell* presShell = GetPresShell();
     if (changed && presShell) {
       if (nsPresContext* presContext = presShell->GetPresContext()) {
         presContext->UserFontSetUpdated();
diff --git a/dom/base/Document.h b/dom/base/Document.h
index 65a790f363b8..385b288d3f3b 100644
--- a/dom/base/Document.h
+++ b/dom/base/Document.h
@@ -1244,9 +1244,8 @@ class Document : public nsINode,
    * method is responsible for calling BeginObservingDocument() on the
    * presshell if the presshell should observe document mutations.
    */
-  already_AddRefed<PresShell> CreatePresShell(
-      nsPresContext* aContext, nsViewManager* aViewManager,
-      UniquePtr<ServoStyleSet> aStyleSet);
+  already_AddRefed<PresShell> CreatePresShell(nsPresContext* aContext,
+                                              nsViewManager* aViewManager);
   void DeletePresShell();
 
   PresShell* GetPresShell() const {
@@ -1631,6 +1630,14 @@ class Document : public nsINode,
   // Get the "head" element in the sense of document.head.
   HTMLSharedElement* GetHead();
 
+  ServoStyleSet* StyleSetForPresShellOrMediaQueryEvaluation() const {
+    return mStyleSet.get();
+  }
+
+  // Whether we filled the style set with any style sheet. Only meant to be used
+  // from DocumentOrShadowRoot::Traverse.
+  bool StyleSetFilled() const { return mStyleSetFilled; }
+
   /**
    * Accessors to the collection of stylesheets owned by this document.
    * Style sheets are ordered, most significant last.
@@ -3784,7 +3791,14 @@ class Document : public nsINode,
   void RemoveStyleSheetsFromStyleSets(
       const nsTArray<RefPtr<StyleSheet>>& aSheets, SheetType aType);
   void ResetStylesheetsToURI(nsIURI* aURI);
-  void FillStyleSet(ServoStyleSet* aStyleSet);
+  void FillStyleSet();
+  void FillStyleSetUserAndUASheets();
+  void FillStyleSetDocumentSheets();
+  void CompatibilityModeChanged();
+  bool NeedsQuirksSheet() const {
+    // SVG documents never load quirk.css.
+    return mCompatMode == eCompatibility_NavQuirks && !IsSVGDocument();
+  }
   void AddStyleSheetToStyleSets(StyleSheet* aSheet);
   void RemoveStyleSheetFromStyleSets(StyleSheet* aSheet);
   void NotifyStyleSheetAdded(StyleSheet* aSheet, bool aDocumentSheet);
@@ -3808,6 +3822,7 @@ class Document : public nsINode,
   // Lazy-initialization to have mDocGroup initialized in prior to the
   // SelectorCaches.
   UniquePtr<SelectorCache> mSelectorCache;
+  UniquePtr<ServoStyleSet> mStyleSet;
 
  protected:
   friend class nsDocumentOnStack;
@@ -4165,10 +4180,12 @@ class Document : public nsINode,
 
   bool mNeedsReleaseAfterStackRefCntRelease : 1;
 
-  // Whether we have filled our pres shell's style set with the document's
-  // additional sheets and sheets from the nsStyleSheetService.
+  // Whether we have filled our style set with all the stylesheets.
   bool mStyleSetFilled : 1;
 
+  // Whether we have a quirks mode stylesheet in the style set.
+  bool mQuirkSheetAdded : 1;
+
   // Keeps track of whether we have a pending
   // 'style-sheet-applicable-state-changed' notification.
   bool mSSApplicableStateNotificationPending : 1;
diff --git a/dom/base/DocumentOrShadowRoot.cpp b/dom/base/DocumentOrShadowRoot.cpp
index 1770f6689ca7..0f005740c112 100644
--- a/dom/base/DocumentOrShadowRoot.cpp
+++ b/dom/base/DocumentOrShadowRoot.cpp
@@ -588,6 +588,29 @@ nsRadioGroupStruct* DocumentOrShadowRoot::GetOrCreateRadioGroup(
 
 void DocumentOrShadowRoot::Traverse(DocumentOrShadowRoot* tmp,
                                     nsCycleCollectionTraversalCallback& cb) {
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mStyleSheets)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mDOMStyleSheets)
+  for (StyleSheet* sheet : tmp->mStyleSheets) {
+    if (!sheet->IsApplicable()) {
+      continue;
+    }
+    // The style set or mServoStyles keep more references to it if the sheet is
+    // applicable.
+    if (tmp->mKind == Kind::ShadowRoot) {
+      NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mServoStyles->sheets[i]");
+      cb.NoteXPCOMChild(sheet);
+    } else if (tmp->AsNode().AsDocument()->StyleSetFilled()) {
+      NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(
+          cb, "mStyleSet->mStyleSheets[SheetType::Author][i]");
+      cb.NoteXPCOMChild(sheet);
+      NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(
+          cb, "mStyleSet->mRawSet.stylist.stylesheets.author[i]");
+      cb.NoteXPCOMChild(sheet);
+    }
+  }
+  for (auto iter = tmp->mIdentifierMap.ConstIter(); !iter.Done(); iter.Next()) {
+    iter.Get()->Traverse(&cb);
+  }
   for (auto iter = tmp->mRadioGroups.Iter(); !iter.Done(); iter.Next()) {
     nsRadioGroupStruct* radioGroup = iter.UserData();
     NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(
@@ -604,6 +627,8 @@ void DocumentOrShadowRoot::Traverse(DocumentOrShadowRoot* tmp,
 }
 
 void DocumentOrShadowRoot::Unlink(DocumentOrShadowRoot* tmp) {
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mDOMStyleSheets)
+  tmp->mIdentifierMap.Clear();
   tmp->mRadioGroups.Clear();
 }
 
diff --git a/dom/base/ShadowRoot.cpp b/dom/base/ShadowRoot.cpp
index 51d751c735c6..ab778ebdef62 100644
--- a/dom/base/ShadowRoot.cpp
+++ b/dom/base/ShadowRoot.cpp
@@ -30,19 +30,6 @@ using namespace mozilla::dom;
 NS_IMPL_CYCLE_COLLECTION_CLASS(ShadowRoot)
 
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(ShadowRoot, DocumentFragment)
-  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mStyleSheets)
-  for (StyleSheet* sheet : tmp->mStyleSheets) {
-    // mServoStyles keeps another reference to it if applicable.
-    if (sheet->IsApplicable()) {
-      MOZ_ASSERT(tmp->mServoStyles);
-      NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mServoStyles->sheets[i]");
-      cb.NoteXPCOMChild(sheet);
-    }
-  }
-  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mDOMStyleSheets)
-  for (auto iter = tmp->mIdentifierMap.ConstIter(); !iter.Done(); iter.Next()) {
-    iter.Get()->Traverse(&cb);
-  }
   DocumentOrShadowRoot::Traverse(tmp, cb);
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
 
@@ -50,8 +37,6 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(ShadowRoot)
   if (tmp->GetHost()) {
     tmp->GetHost()->RemoveMutationObserver(tmp);
   }
-  NS_IMPL_CYCLE_COLLECTION_UNLINK(mDOMStyleSheets)
-  tmp->mIdentifierMap.Clear();
   DocumentOrShadowRoot::Unlink(tmp);
 NS_IMPL_CYCLE_COLLECTION_UNLINK_END_INHERITED(DocumentFragment)
 
diff --git a/dom/canvas/CanvasRenderingContext2D.cpp b/dom/canvas/CanvasRenderingContext2D.cpp
index e5b7dc50e1c9..4431843aef65 100644
--- a/dom/canvas/CanvasRenderingContext2D.cpp
+++ b/dom/canvas/CanvasRenderingContext2D.cpp
@@ -20,6 +20,7 @@
 #include "SVGObserverUtils.h"
 #include "nsPresContext.h"
 #include "nsIPresShell.h"
+#include "nsIPresShellInlines.h"
 
 #include "nsIInterfaceRequestorUtils.h"
 #include "nsIFrame.h"
diff --git a/dom/html/nsHTMLDocument.cpp b/dom/html/nsHTMLDocument.cpp
index e8e9694d7145..2cdbab0233c8 100644
--- a/dom/html/nsHTMLDocument.cpp
+++ b/dom/html/nsHTMLDocument.cpp
@@ -509,7 +509,7 @@ nsresult nsHTMLDocument::StartDocumentLoad(const char* aCommand,
   if (!viewSource && xhtml) {
     // We're parsing XHTML as XML, remember that.
     mType = eXHTML;
-    mCompatMode = eCompatibility_FullStandards;
+    SetCompatibilityMode(eCompatibility_FullStandards);
     loadAsHtml5 = false;
   }
 
@@ -528,8 +528,6 @@ nsresult nsHTMLDocument::StartDocumentLoad(const char* aCommand,
     }
   }
 
-  CSSLoader()->SetCompatibilityMode(mCompatMode);
-
   nsresult rv = Document::StartDocumentLoad(aCommand, aChannel, aLoadGroup,
                                             aContainer, aDocListener, aReset);
   if (NS_FAILED(rv)) {
@@ -782,10 +780,7 @@ void nsHTMLDocument::SetCompatibilityMode(nsCompatibility aMode) {
     return;
   }
   mCompatMode = aMode;
-  CSSLoader()->SetCompatibilityMode(mCompatMode);
-  if (nsPresContext* pc = GetPresContext()) {
-    pc->CompatibilityModeChanged();
-  }
+  CompatibilityModeChanged();
 }
 
 bool nsHTMLDocument::UseWidthDeviceWidthFallbackViewport() const {
diff --git a/editor/libeditor/HTMLAnonymousNodeEditor.cpp b/editor/libeditor/HTMLAnonymousNodeEditor.cpp
index cebd6638daa0..e74fa15ef4e9 100644
--- a/editor/libeditor/HTMLAnonymousNodeEditor.cpp
+++ b/editor/libeditor/HTMLAnonymousNodeEditor.cpp
@@ -27,6 +27,7 @@
 #include "nsIHTMLObjectResizer.h"
 #include "nsStubMutationObserver.h"
 #include "nsINode.h"
+#include "nsIPresShellInlines.h"
 #include "nsISupportsImpl.h"
 #include "nsISupportsUtils.h"
 #include "nsLiteralString.h"
diff --git a/layout/base/PresShell.cpp b/layout/base/PresShell.cpp
index a6dcc33ce2d6..c35d2ef8370c 100644
--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -897,7 +897,6 @@ PresShell::~PresShell() {
   MOZ_ASSERT(mAllocatedPointers.IsEmpty(),
              "Some pres arena objects were not freed");
 
-  mStyleSet = nullptr;
   mFrameManager = nullptr;
   mFrameConstructor = nullptr;
 
@@ -911,8 +910,7 @@ PresShell::~PresShell() {
  * calls AddRef() on us.
  */
 void PresShell::Init(Document* aDocument, nsPresContext* aPresContext,
-                     nsViewManager* aViewManager,
-                     UniquePtr<ServoStyleSet> aStyleSet) {
+                     nsViewManager* aViewManager) {
   MOZ_ASSERT(aDocument, "null ptr");
   MOZ_ASSERT(aPresContext, "null ptr");
   MOZ_ASSERT(aViewManager, "null ptr");
@@ -944,16 +942,7 @@ void PresShell::Init(Document* aDocument, nsPresContext* aPresContext,
   mPresContext = aPresContext;
   mPresContext->AttachShell(this);
 
-  // Now we can initialize the style set. Make sure to set the member before
-  // calling Init, since various subroutines need to find the style set off
-  // the PresContext during initialization.
-  mStyleSet = std::move(aStyleSet);
-  mStyleSet->Init(aPresContext);
-
-  // Notify our prescontext that it now has a compatibility mode.  Note that
-  // this MUST happen after we set up our style set but before we create any
-  // frames.
-  mPresContext->CompatibilityModeChanged();
+  mPresContext->DeviceContext()->InitFontCache();
 
   // Add the preference style sheet.
   UpdatePreferenceStyles();
@@ -1276,8 +1265,7 @@ void PresShell::Destroy() {
 
   // release our pref style sheet, if we have one still
   //
-  // FIXME(emilio): Why do we need to do this? The stylist is getting nixed with
-  // us anyway.
+  // TODO(emilio): Should we move the preference sheet tracking to the Document?
   RemovePreferenceStyles();
 
   mIsDestroying = true;
@@ -1310,7 +1298,6 @@ void PresShell::Destroy() {
     mViewManager = nullptr;
   }
 
-  mStyleSet->BeginShutdown();
   nsRefreshDriver* rd = GetPresContext()->RefreshDriver();
 
   // This shell must be removed from the document before the frame
@@ -1361,9 +1348,6 @@ void PresShell::Destroy() {
     weakFrame->Clear(this);
   }
 
-  // Let the style set do its cleanup.
-  mStyleSet->Shutdown();
-
   if (mPresContext) {
     // We hold a reference to the pres context, and it holds a weak link back
     // to us. To avoid the pres context having a dangling reference, set its
@@ -1410,8 +1394,8 @@ nsRefreshDriver* nsIPresShell::GetRefreshDriver() const {
 }
 
 void nsIPresShell::SetAuthorStyleDisabled(bool aStyleDisabled) {
-  if (aStyleDisabled != mStyleSet->GetAuthorStyleDisabled()) {
-    mStyleSet->SetAuthorStyleDisabled(aStyleDisabled);
+  if (aStyleDisabled != StyleSet()->GetAuthorStyleDisabled()) {
+    StyleSet()->SetAuthorStyleDisabled(aStyleDisabled);
     ApplicableStylesChanged();
 
     nsCOMPtr<nsIObserverService> observerService =
@@ -1424,7 +1408,7 @@ void nsIPresShell::SetAuthorStyleDisabled(bool aStyleDisabled) {
 }
 
 bool nsIPresShell::GetAuthorStyleDisabled() const {
-  return mStyleSet->GetAuthorStyleDisabled();
+  return StyleSet()->GetAuthorStyleDisabled();
 }
 
 void nsIPresShell::UpdatePreferenceStyles() {
@@ -1463,13 +1447,13 @@ void nsIPresShell::UpdatePreferenceStyles() {
   // it to be modifiable from devtools and similar, see bugs 1239336 and
   // 1436782. I think it conceptually should be a user sheet, and could be
   // without too much trouble I'd think.
-  mStyleSet->AppendStyleSheet(SheetType::Agent, newPrefSheet);
+  StyleSet()->AppendStyleSheet(SheetType::Agent, newPrefSheet);
   mPrefStyleSheet = newPrefSheet;
 }
 
 void nsIPresShell::RemovePreferenceStyles() {
   if (mPrefStyleSheet) {
-    mStyleSet->RemoveStyleSheet(SheetType::Agent, mPrefStyleSheet);
+    StyleSet()->RemoveStyleSheet(SheetType::Agent, mPrefStyleSheet);
     mPrefStyleSheet = nullptr;
   }
 }
@@ -1496,14 +1480,14 @@ void nsIPresShell::AddUserSheet(StyleSheet* aSheet) {
   // Assert that all of userSheets (except for the last, new element) matches up
   // with what's in the style set.
   for (size_t i = 0; i < index; ++i) {
-    MOZ_ASSERT(mStyleSet->StyleSheetAt(SheetType::User, i) == userSheets[i]);
+    MOZ_ASSERT(StyleSet()->StyleSheetAt(SheetType::User, i) == userSheets[i]);
   }
 
-  if (index == static_cast<size_t>(mStyleSet->SheetCount(SheetType::User))) {
-    mStyleSet->AppendStyleSheet(SheetType::User, aSheet);
+  if (index == static_cast<size_t>(StyleSet()->SheetCount(SheetType::User))) {
+    StyleSet()->AppendStyleSheet(SheetType::User, aSheet);
   } else {
-    StyleSheet* ref = mStyleSet->StyleSheetAt(SheetType::User, index);
-    mStyleSet->InsertStyleSheetBefore(SheetType::User, aSheet, ref);
+    StyleSheet* ref = StyleSet()->StyleSheetAt(SheetType::User, index);
+    StyleSet()->InsertStyleSheetBefore(SheetType::User, aSheet, ref);
   }
 
   ApplicableStylesChanged();
@@ -1512,7 +1496,7 @@ void nsIPresShell::AddUserSheet(StyleSheet* aSheet) {
 void nsIPresShell::AddAgentSheet(StyleSheet* aSheet) {
   // Make sure this does what nsDocumentViewer::CreateStyleSet does
   // wrt ordering.
-  mStyleSet->AppendStyleSheet(SheetType::Agent, aSheet);
+  StyleSet()->AppendStyleSheet(SheetType::Agent, aSheet);
   ApplicableStylesChanged();
 }
 
@@ -1521,16 +1505,17 @@ void nsIPresShell::AddAuthorSheet(StyleSheet* aSheet) {
   // ones added with the StyleSheetService.
   StyleSheet* firstAuthorSheet = mDocument->GetFirstAdditionalAuthorSheet();
   if (firstAuthorSheet) {
-    mStyleSet->InsertStyleSheetBefore(SheetType::Doc, aSheet, firstAuthorSheet);
+    StyleSet()->InsertStyleSheetBefore(SheetType::Doc, aSheet,
+                                       firstAuthorSheet);
   } else {
-    mStyleSet->AppendStyleSheet(SheetType::Doc, aSheet);
+    StyleSet()->AppendStyleSheet(SheetType::Doc, aSheet);
   }
 
   ApplicableStylesChanged();
 }
 
 void nsIPresShell::RemoveSheet(SheetType aType, StyleSheet* aSheet) {
-  mStyleSet->RemoveStyleSheet(aType, aSheet);
+  StyleSet()->RemoveStyleSheet(aType, aSheet);
   ApplicableStylesChanged();
 }
 
@@ -4089,7 +4074,7 @@ void PresShell::DoFlushPendingNotifications(mozilla::ChangesToFlush aFlush) {
     if (MOZ_LIKELY(!mIsDestroying)) {
       // Now that we have flushed media queries, update the rules before looking
       // up @font-face / @counter-style / @font-feature-values rules.
-      mStyleSet->UpdateStylistIfNeeded();
+      StyleSet()->UpdateStylistIfNeeded();
 
       // Flush any pending update of the user font set, since that could
       // cause style changes (for updating ex/ch units, and to cause a
@@ -4234,7 +4219,7 @@ void PresShell::DocumentStatesChanged(Document* aDocument,
   MOZ_ASSERT(!aStateMask.IsEmpty());
 
   if (mDidInitialize) {
-    mStyleSet->InvalidateStyleForDocumentStateChanges(aStateMask);
+    StyleSet()->InvalidateStyleForDocumentStateChanges(aStateMask);
   }
 
   if (aStateMask.HasState(NS_DOCUMENT_STATE_WINDOW_INACTIVE)) {
@@ -5904,7 +5889,7 @@ class nsAutoNotifyDidPaint {
 };
 
 void nsIPresShell::RecordShadowStyleChange(ShadowRoot& aShadowRoot) {
-  mStyleSet->RecordShadowStyleChange(aShadowRoot);
+  StyleSet()->RecordShadowStyleChange(aShadowRoot);
   ApplicableStylesChanged();
 }
 
@@ -8798,14 +8783,14 @@ bool PresShell::IsDisplayportSuppressed() {
 
 nsresult PresShell::GetAgentStyleSheets(nsTArray<RefPtr<StyleSheet>>& aSheets) {
   aSheets.Clear();
-  int32_t sheetCount = mStyleSet->SheetCount(SheetType::Agent);
+  int32_t sheetCount = StyleSet()->SheetCount(SheetType::Agent);
 
   if (!aSheets.SetCapacity(sheetCount, fallible)) {
     return NS_ERROR_OUT_OF_MEMORY;
   }
 
   for (int32_t i = 0; i < sheetCount; ++i) {
-    StyleSheet* sheet = mStyleSet->StyleSheetAt(SheetType::Agent, i);
+    StyleSheet* sheet = StyleSet()->StyleSheetAt(SheetType::Agent, i);
     aSheets.AppendElement(sheet);
   }
 
@@ -8814,15 +8799,15 @@ nsresult PresShell::GetAgentStyleSheets(nsTArray<RefPtr<StyleSheet>>& aSheets) {
 
 nsresult PresShell::SetAgentStyleSheets(
     const nsTArray<RefPtr<StyleSheet>>& aSheets) {
-  return mStyleSet->ReplaceSheets(SheetType::Agent, aSheets);
+  return StyleSet()->ReplaceSheets(SheetType::Agent, aSheets);
 }
 
 nsresult PresShell::AddOverrideStyleSheet(StyleSheet* aSheet) {
-  return mStyleSet->AppendStyleSheet(SheetType::Override, aSheet);
+  return StyleSet()->AppendStyleSheet(SheetType::Override, aSheet);
 }
 
 nsresult PresShell::RemoveOverrideStyleSheet(StyleSheet* aSheet) {
-  return mStyleSet->RemoveStyleSheet(SheetType::Override, aSheet);
+  return StyleSet()->RemoveStyleSheet(SheetType::Override, aSheet);
 }
 
 static void FreezeElement(nsISupports* aSupports, void* /* unused */) {
@@ -9835,42 +9820,6 @@ FindTopFrame(nsIFrame* aRoot)
 
 #ifdef DEBUG
 
-static void CopySheetsIntoClone(ServoStyleSet* aSet, ServoStyleSet* aClone) {
-  int32_t i, n = aSet->SheetCount(SheetType::Override);
-  for (i = 0; i < n; i++) {
-    StyleSheet* ss = aSet->StyleSheetAt(SheetType::Override, i);
-    if (ss) aClone->AppendStyleSheet(SheetType::Override, ss);
-  }
-
-  // The document expects to insert document stylesheets itself
-#  if 0
-  n = aSet->SheetCount(SheetType::Doc);
-  for (i = 0; i < n; i++) {
-    StyleSheet* ss = aSet->StyleSheetAt(SheetType::Doc, i);
-    if (ss)
-      aClone->AddDocStyleSheet(ss, mDocument);
-  }
-#  endif
-
-  n = aSet->SheetCount(SheetType::User);
-  for (i = 0; i < n; i++) {
-    StyleSheet* ss = aSet->StyleSheetAt(SheetType::User, i);
-    if (ss) aClone->AppendStyleSheet(SheetType::User, ss);
-  }
-
-  n = aSet->SheetCount(SheetType::Agent);
-  for (i = 0; i < n; i++) {
-    StyleSheet* ss = aSet->StyleSheetAt(SheetType::Agent, i);
-    if (ss) aClone->AppendStyleSheet(SheetType::Agent, ss);
-  }
-}
-
-UniquePtr<ServoStyleSet> nsIPresShell::CloneStyleSet(ServoStyleSet* aSet) {
-  auto clone = MakeUnique<ServoStyleSet>();
-  CopySheetsIntoClone(aSet, clone.get());
-  return clone;
-}
-
 // After an incremental reflow, we verify the correctness by doing a
 // full reflow into a fresh frame tree.
 bool nsIPresShell::VerifyIncrementalReflow() {
@@ -9917,13 +9866,9 @@ bool nsIPresShell::VerifyIncrementalReflow() {
   // presentation context.
   cx->SetVisibleArea(mPresContext->GetVisibleArea());
 
-  // Create a new presentation shell to view the document. Use the
-  // exact same style information that this document has.
-  UniquePtr<ServoStyleSet> newSet = CloneStyleSet(StyleSet());
-
-  RefPtr<PresShell> presShell =
-      mDocument->CreatePresShell(cx, vm, std::move(newSet));
+  RefPtr<PresShell> presShell = mDocument->CreatePresShell(cx, vm);
   NS_ENSURE_TRUE(presShell, false);
+
   // Note that after we create the shell, we must make sure to destroy it
   presShell->SetVerifyReflowEnable(
       false);  // turn off verify reflow while we're
@@ -10003,9 +9948,9 @@ void PresShell::ListComputedStyles(FILE* out, int32_t aIndent) {
 }
 
 void PresShell::ListStyleSheets(FILE* out, int32_t aIndent) {
-  int32_t sheetCount = mStyleSet->SheetCount(SheetType::Doc);
+  int32_t sheetCount = StyleSet()->SheetCount(SheetType::Doc);
   for (int32_t i = 0; i < sheetCount; ++i) {
-    mStyleSet->StyleSheetAt(SheetType::Doc, i)->List(out, aIndent);
+    StyleSet()->StyleSheetAt(SheetType::Doc, i)->List(out, aIndent);
     fputs("\n", out);
   }
 }
@@ -10909,10 +10854,6 @@ nsresult nsIPresShell::HasRuleProcessorUsedByMultipleStyleSets(
 
 void nsIPresShell::NotifyStyleSheetServiceSheetAdded(StyleSheet* aSheet,
                                                      uint32_t aSheetType) {
-  if (!mStyleSet) {
-    return;
-  }
-
   switch (aSheetType) {
     case nsIStyleSheetService::AGENT_SHEET:
       AddAgentSheet(aSheet);
@@ -10931,10 +10872,6 @@ void nsIPresShell::NotifyStyleSheetServiceSheetAdded(StyleSheet* aSheet,
 
 void nsIPresShell::NotifyStyleSheetServiceSheetRemoved(StyleSheet* aSheet,
                                                        uint32_t aSheetType) {
-  if (!mStyleSet) {
-    return;
-  }
-
   RemoveSheet(ToSheetType(aSheetType), aSheet);
 }
 
diff --git a/layout/base/PresShell.h b/layout/base/PresShell.h
index 9addba1a7ea7..ddaffa530620 100644
--- a/layout/base/PresShell.h
+++ b/layout/base/PresShell.h
@@ -64,6 +64,7 @@ class PresShell final : public nsIPresShell,
                         public nsIObserver,
                         public nsSupportsWeakReference {
   typedef layers::FocusTarget FocusTarget;
+  typedef dom::Element Element;
 
  public:
   PresShell();
@@ -73,8 +74,7 @@ class PresShell final : public nsIPresShell,
 
   static bool AccessibleCaretEnabled(nsIDocShell* aDocShell);
 
-  void Init(Document* aDocument, nsPresContext* aPresContext,
-            nsViewManager* aViewManager, UniquePtr<ServoStyleSet> aStyleSet);
+  void Init(Document*, nsPresContext*, nsViewManager*);
   void Destroy() override;
 
   NS_IMETHOD GetSelectionFromScript(RawSelectionType aRawSelectionType,
diff --git a/layout/base/nsDocumentViewer.cpp b/layout/base/nsDocumentViewer.cpp
index 72bb0b274c45..a41e38f0b4fb 100644
--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -726,12 +726,8 @@ nsresult nsDocumentViewer::InitPresentationStuff(bool aDoInitialReflow) {
 
   NS_ASSERTION(!mPresShell, "Someone should have destroyed the presshell!");
 
-  // Create the style set...
-  UniquePtr<ServoStyleSet> styleSet = CreateStyleSet(mDocument);
-
   // Now make the shell for the document
-  mPresShell = mDocument->CreatePresShell(mPresContext, mViewManager,
-                                          std::move(styleSet));
+  mPresShell = mDocument->CreatePresShell(mPresContext, mViewManager);
   if (!mPresShell) {
     return NS_ERROR_FAILURE;
   }
@@ -2291,81 +2287,6 @@ nsDocumentViewer::RequestWindowClose(bool* aCanClose) {
   return NS_OK;
 }
 
-UniquePtr<ServoStyleSet> nsDocumentViewer::CreateStyleSet(Document* aDocument) {
-  // Make sure this does the same thing as PresShell::Add{User,Agent}Sheet wrt
-  // ordering.
-
-  // The document will fill in the document sheets when we create the presshell
-  auto cache = nsLayoutStylesheetCache::Singleton();
-  nsStyleSheetService* sheetService = nsStyleSheetService::GetInstance();
-
-  MOZ_ASSERT(sheetService,
-             "should never be creating a StyleSet after the style sheet "
-             "service has gone");
-
-  auto styleSet = MakeUnique<ServoStyleSet>();
-
-  // User sheets
-
-  for (StyleSheet* sheet : *sheetService->UserStyleSheets()) {
-    styleSet->AppendStyleSheet(SheetType::User, sheet);
-  }
-
-  StyleSheet* sheet = nsContentUtils::IsInChromeDocshell(aDocument)
-                          ? cache->GetUserChromeSheet()
-                          : cache->GetUserContentSheet();
-
-  if (sheet) {
-    styleSet->AppendStyleSheet(SheetType::User, sheet);
-  }
-
-  // Agent sheets
-
-  styleSet->AppendStyleSheet(SheetType::Agent, cache->UASheet());
-
-  if (MOZ_LIKELY(mDocument->NodeInfoManager()->MathMLEnabled())) {
-    styleSet->AppendStyleSheet(SheetType::Agent, cache->MathMLSheet());
-  }
-
-  if (MOZ_LIKELY(mDocument->NodeInfoManager()->SVGEnabled())) {
-    styleSet->AppendStyleSheet(SheetType::Agent, cache->SVGSheet());
-  }
-
-  styleSet->AppendStyleSheet(SheetType::Agent, cache->HTMLSheet());
-
-  // We don't add quirk.css here; nsPresContext::CompatibilityModeChanged will
-  // append it if needed.
-
-  if (nsLayoutUtils::ShouldUseNoFramesSheet(aDocument)) {
-    styleSet->AppendStyleSheet(SheetType::Agent, cache->NoFramesSheet());
-  }
-
-  if (nsLayoutUtils::ShouldUseNoScriptSheet(aDocument)) {
-    styleSet->AppendStyleSheet(SheetType::Agent, cache->NoScriptSheet());
-  }
-
-  styleSet->AppendStyleSheet(SheetType::Agent, cache->CounterStylesSheet());
-
-  // Load the minimal XUL rules for scrollbars and a few other XUL things
-  // that non-XUL (typically HTML) documents commonly use.
-  styleSet->AppendStyleSheet(SheetType::Agent, cache->MinimalXULSheet());
-
-  // Only load the full XUL sheet if we'll need it.
-  if (aDocument->LoadsFullXULStyleSheetUpFront()) {
-    styleSet->AppendStyleSheet(SheetType::Agent, cache->XULSheet());
-  }
-
-  styleSet->AppendStyleSheet(SheetType::Agent, cache->FormsSheet());
-  styleSet->AppendStyleSheet(SheetType::Agent, cache->ScrollbarsSheet());
-  styleSet->AppendStyleSheet(SheetType::Agent, cache->PluginProblemSheet());
-
-  for (StyleSheet* sheet : *sheetService->AgentStyleSheets()) {
-    styleSet->AppendStyleSheet(SheetType::Agent, sheet);
-  }
-
-  return styleSet;
-}
-
 NS_IMETHODIMP
 nsDocumentViewer::ClearHistoryEntry() {
   if (mDocument) {
diff --git a/layout/base/nsIDocumentViewerPrint.h b/layout/base/nsIDocumentViewerPrint.h
index 1e8b369bc7e0..5a36f1f02694 100644
--- a/layout/base/nsIDocumentViewerPrint.h
+++ b/layout/base/nsIDocumentViewerPrint.h
@@ -39,12 +39,6 @@ class nsIDocumentViewerPrint : public nsISupports {
   virtual void SetIsPrintPreview(bool aIsPrintPreview) = 0;
   virtual bool GetIsPrintPreview() = 0;
 
-  // The style set returned by CreateStyleSet is in the middle of an
-  // update batch so that the caller can add sheets to it if needed.
-  // Callers should call EndUpdate() on it when ready to use.
-  virtual mozilla::UniquePtr<mozilla::ServoStyleSet> CreateStyleSet(
-      mozilla::dom::Document* aDocument) = 0;
-
   /**
    * This is used by nsPagePrintTimer to make nsDocumentViewer::Destroy()
    * a no-op until printing is finished.  That prevents the nsDocumentViewer
@@ -82,8 +76,6 @@ NS_DEFINE_STATIC_IID_ACCESSOR(nsIDocumentViewerPrint,
   bool GetIsPrinting() override;                                \
   void SetIsPrintPreview(bool aIsPrintPreview) override;        \
   bool GetIsPrintPreview() override;                            \
-  mozilla::UniquePtr<mozilla::ServoStyleSet> CreateStyleSet(    \
-      mozilla::dom::Document* aDocument) override;              \
   void IncrementDestroyBlockedCount() override;                 \
   void DecrementDestroyBlockedCount() override;                 \
   void OnDonePrinting() override;                               \
diff --git a/layout/base/nsIPresShell.h b/layout/base/nsIPresShell.h
index 78c4cf70b037..12401184c50d 100644
--- a/layout/base/nsIPresShell.h
+++ b/layout/base/nsIPresShell.h
@@ -272,7 +272,7 @@ class nsIPresShell : public nsStubDocumentObserver {
   }
 #endif
 
-  mozilla::ServoStyleSet* StyleSet() const { return mStyleSet.get(); }
+  inline mozilla::ServoStyleSet* StyleSet() const;
 
   nsCSSFrameConstructor* FrameConstructor() const {
     return mFrameConstructor.get();
@@ -1778,8 +1778,6 @@ class nsIPresShell : public nsStubDocumentObserver {
       FrameMetrics::ScrollOffsetUpdateType aUpdateType);
 
 #ifdef DEBUG
-  mozilla::UniquePtr<mozilla::ServoStyleSet> CloneStyleSet(
-      mozilla::ServoStyleSet*);
   bool VerifyIncrementalReflow();
   void DoVerifyReflow();
   void VerifyHasDirtyRootAncestor(nsIFrame* aFrame);
@@ -1857,9 +1855,8 @@ class nsIPresShell : public nsStubDocumentObserver {
   // we must share ownership.
   RefPtr<Document> mDocument;
   RefPtr<nsPresContext> mPresContext;
-  // mStyleSet owns it but we maintain a ref, may be null
+  // The document's style set owns it but we maintain a ref, may be null.
   RefPtr<mozilla::StyleSheet> mPrefStyleSheet;
-  mozilla::UniquePtr<mozilla::ServoStyleSet> mStyleSet;
   mozilla::UniquePtr<nsCSSFrameConstructor> mFrameConstructor;
   nsViewManager* mViewManager;  // [WEAK] docViewer owns it so I don't have to
   nsPresArena<8192> mFrameArena;
diff --git a/layout/base/nsIPresShellInlines.h b/layout/base/nsIPresShellInlines.h
index 355780518a57..8857a7b05a65 100644
--- a/layout/base/nsIPresShellInlines.h
+++ b/layout/base/nsIPresShellInlines.h
@@ -9,6 +9,7 @@
 
 #include "mozilla/PresShell.h"
 #include "mozilla/dom/Document.h"
+#include "mozilla/dom/Element.h"
 
 void nsIPresShell::SetNeedLayoutFlush() {
   mNeedLayoutFlush = true;
@@ -54,6 +55,10 @@ void nsIPresShell::SetNeedThrottledAnimationFlush() {
   }
 }
 
+mozilla::ServoStyleSet* nsIPresShell::StyleSet() const {
+  return mDocument->StyleSetForPresShellOrMediaQueryEvaluation();
+}
+
 namespace mozilla {
 
 /* static */
diff --git a/layout/base/nsPresContext.cpp b/layout/base/nsPresContext.cpp
index 8f96cee5d3d2..ff9d853f5136 100644
--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -847,44 +847,6 @@ nsRootPresContext* nsPresContext::GetRootPresContext() {
   return pc->IsRoot() ? static_cast<nsRootPresContext*>(pc) : nullptr;
 }
 
-void nsPresContext::CompatibilityModeChanged() {
-  if (!mShell) {
-    return;
-  }
-
-  ServoStyleSet* styleSet = mShell->StyleSet();
-  styleSet->CompatibilityModeChanged();
-
-  mShell->EnsureStyleFlush();
-
-  if (mDocument->IsSVGDocument()) {
-    // SVG documents never load quirk.css.
-    return;
-  }
-
-  bool needsQuirkSheet = CompatibilityMode() == eCompatibility_NavQuirks;
-  if (mQuirkSheetAdded == needsQuirkSheet) {
-    return;
-  }
-
-  auto cache = nsLayoutStylesheetCache::Singleton();
-  StyleSheet* sheet = cache->QuirkSheet();
-
-  if (needsQuirkSheet) {
-    // quirk.css needs to come after html.css; we just keep it at the end.
-    DebugOnly<nsresult> rv =
-        styleSet->AppendStyleSheet(SheetType::Agent, sheet);
-    NS_WARNING_ASSERTION(NS_SUCCEEDED(rv), "failed to insert quirk.css");
-  } else {
-    DebugOnly<nsresult> rv =
-        styleSet->RemoveStyleSheet(SheetType::Agent, sheet);
-    NS_WARNING_ASSERTION(NS_SUCCEEDED(rv), "failed to remove quirk.css");
-  }
-
-  mQuirkSheetAdded = needsQuirkSheet;
-  mShell->ApplicableStylesChanged();
-}
-
 // Helper function for setting Anim Mode on image
 static void SetImgAnimModeOnImgReq(imgIRequest* aImgReq, uint16_t aMode) {
   if (aImgReq) {
diff --git a/layout/base/nsPresContext.h b/layout/base/nsPresContext.h
index 1e0a854f9343..9a92e22c9ab4 100644
--- a/layout/base/nsPresContext.h
+++ b/layout/base/nsPresContext.h
@@ -294,11 +294,6 @@ class nsPresContext : public nsISupports,
    */
   nsCompatibility CompatibilityMode() const;
 
-  /**
-   * Notify the context that the document's compatibility mode has changed
-   */
-  void CompatibilityModeChanged();
-
   /**
    * Access the image animation mode for this context
    */
diff --git a/layout/base/nsPresContextInlines.h b/layout/base/nsPresContextInlines.h
index 83acb145461e..ed6c696d6ca8 100644
--- a/layout/base/nsPresContextInlines.h
+++ b/layout/base/nsPresContextInlines.h
@@ -4,18 +4,18 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-#ifndef nsIPresContextInlines_h
-#define nsIPresContextInlines_h
+#ifndef nsPresContextInlines_h
+#define nsPresContextInlines_h
 
 #include "mozilla/PresShell.h"
 #include "nsCSSFrameConstructor.h"
 
 inline mozilla::ServoStyleSet* nsPresContext::StyleSet() const {
-  return GetPresShell()->StyleSet();
+  return mDocument->StyleSetForPresShellOrMediaQueryEvaluation();
 }
 
 inline nsCSSFrameConstructor* nsPresContext::FrameConstructor() {
   return PresShell()->FrameConstructor();
 }
 
-#endif  // #ifndef nsIPresContextInlines_h
+#endif  // #ifndef nsPresContextInlines_h
diff --git a/layout/base/nsStyleSheetService.cpp b/layout/base/nsStyleSheetService.cpp
index 61fea4d63d16..f9108761996d 100644
--- a/layout/base/nsStyleSheetService.cpp
+++ b/layout/base/nsStyleSheetService.cpp
@@ -24,6 +24,7 @@
 #include "nsNetUtil.h"
 #include "nsIConsoleService.h"
 #include "nsIObserverService.h"
+#include "nsIPresShellInlines.h"
 #include "nsLayoutStatics.h"
 #include "nsLayoutUtils.h"
 
diff --git a/layout/inspector/InspectorUtils.cpp b/layout/inspector/InspectorUtils.cpp
index 1224bad89790..6393a31ace32 100644
--- a/layout/inspector/InspectorUtils.cpp
+++ b/layout/inspector/InspectorUtils.cpp
@@ -18,6 +18,7 @@
 #include "nsIContentInlines.h"
 #include "mozilla/dom/Document.h"
 #include "nsIDOMWindow.h"
+#include "nsIPresShellInlines.h"
 #include "nsXBLBinding.h"
 #include "nsXBLPrototypeBinding.h"
 #include "nsIMutableArray.h"
diff --git a/layout/printing/nsPrintJob.cpp b/layout/printing/nsPrintJob.cpp
index d9f6d11e46d7..326c62c14999 100644
--- a/layout/printing/nsPrintJob.cpp
+++ b/layout/printing/nsPrintJob.cpp
@@ -2235,11 +2235,8 @@ nsresult nsPrintJob::ReflowPrintObject(const UniquePtr<nsPrintObject>& aPO) {
   rv = aPO->mViewManager->Init(printData->mPrintDC);
   NS_ENSURE_SUCCESS(rv, rv);
 
-  UniquePtr<ServoStyleSet> styleSet =
-      mDocViewerPrint->CreateStyleSet(aPO->mDocument);
-
-  aPO->mPresShell = aPO->mDocument->CreatePresShell(
-      aPO->mPresContext, aPO->mViewManager, std::move(styleSet));
+  aPO->mPresShell =
+      aPO->mDocument->CreatePresShell(aPO->mPresContext, aPO->mViewManager);
   if (!aPO->mPresShell) {
     return NS_ERROR_FAILURE;
   }
diff --git a/layout/style/Loader.cpp b/layout/style/Loader.cpp
index 0771a336a582..07fcb56f9810 100644
--- a/layout/style/Loader.cpp
+++ b/layout/style/Loader.cpp
@@ -369,8 +369,9 @@ Loader::Loader()
 Loader::Loader(DocGroup* aDocGroup) : Loader() { mDocGroup = aDocGroup; }
 
 Loader::Loader(Document* aDocument) : Loader() {
+  MOZ_ASSERT(aDocument, "We should get a valid document from the caller!");
   mDocument = aDocument;
-  MOZ_ASSERT(mDocument, "We should get a valid document from the caller!");
+  mCompatMode = aDocument->GetCompatibilityMode();
 }
 
 Loader::~Loader() {
diff --git a/layout/style/MediaFeatureChange.h b/layout/style/MediaFeatureChange.h
index 7e091316394b..1ec8324a7f26 100644
--- a/layout/style/MediaFeatureChange.h
+++ b/layout/style/MediaFeatureChange.h
@@ -44,6 +44,8 @@ enum class MediaFeatureChangeReason {
 MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(MediaFeatureChangeReason)
 
 struct MediaFeatureChange {
+  static const auto kAllChanges = static_cast<MediaFeatureChangeReason>(~0);
+
   RestyleHint mRestyleHint;
   nsChangeHint mChangeHint;
   MediaFeatureChangeReason mReason;
diff --git a/layout/style/ServoStyleSet.cpp b/layout/style/ServoStyleSet.cpp
index 4f4157a6f4b4..44e5a2d7b0ba 100644
--- a/layout/style/ServoStyleSet.cpp
+++ b/layout/style/ServoStyleSet.cpp
@@ -92,12 +92,15 @@ class MOZ_RAII AutoPrepareTraversal {
 
 }  // namespace mozilla
 
-ServoStyleSet::ServoStyleSet()
-    : mDocument(nullptr),
+ServoStyleSet::ServoStyleSet(Document& aDocument)
+    : mDocument(&aDocument),
       mAuthorStyleDisabled(false),
       mStylistState(StylistState::NotDirty),
       mUserFontSetUpdateGeneration(0),
-      mNeedsRestyleAfterEnsureUniqueInner(false) {}
+      mNeedsRestyleAfterEnsureUniqueInner(false) {
+  PreferenceSheet::EnsureInitialized();
+  mRawSet.reset(Servo_StyleSet_Init(&aDocument));
+}
 
 ServoStyleSet::~ServoStyleSet() {
   for (auto& sheetArray : mSheets) {
@@ -108,41 +111,10 @@ ServoStyleSet::~ServoStyleSet() {
 }
 
 nsPresContext* ServoStyleSet::GetPresContext() {
-  if (!mDocument) {
-    return nullptr;
-  }
-
   return mDocument->GetPresContext();
 }
 
-void ServoStyleSet::Init(nsPresContext* aPresContext) {
-  mDocument = aPresContext->Document();
-  MOZ_ASSERT(GetPresContext() == aPresContext);
-
-  mRawSet.reset(Servo_StyleSet_Init(aPresContext));
-
-  aPresContext->DeviceContext()->InitFontCache();
-
-  // Now that we have an mRawSet, go ahead and notify about whatever stylesheets
-  // we have so far.
-  for (auto& sheetArray : mSheets) {
-    for (auto& sheet : sheetArray) {
-      // There's no guarantee this will create a list on the servo side whose
-      // ordering matches the list that would have been created had all those
-      // sheets been appended/prepended/etc after we had mRawSet. That's okay
-      // because Servo only needs to maintain relative ordering within a sheet
-      // type, which this preserves.
-
-      MOZ_ASSERT(sheet->RawContents(),
-                 "We should only append non-null raw sheets.");
-      Servo_StyleSet_AppendStyleSheet(mRawSet.get(), sheet);
-    }
-  }
-
-  // We added prefilled stylesheets into mRawSet, so the stylist is dirty.
-  // The Stylist should be updated later when necessary.
-  SetStylistStyleSheetsDirty();
-
+void ServoStyleSet::ShellAttachedToDocument() {
   // We may have Shadow DOM style changes that we weren't notified about because
   // the document didn't have a shell, if the ShadowRoot was created in a
   // display: none iframe.
@@ -165,11 +137,10 @@ void EnumerateShadowRoots(const Document& aDoc, const Functor& aCb) {
   }
 }
 
-void ServoStyleSet::Shutdown() {
+void ServoStyleSet::ShellDetachedFromDocument() {
   // Make sure we drop our cached styles before the presshell arena starts going
   // away.
   ClearNonInheritingComputedStyles();
-  mRawSet = nullptr;
   mStyleRuleMap = nullptr;
 }
 
@@ -630,13 +601,11 @@ nsresult ServoStyleSet::AppendStyleSheet(SheetType aType, StyleSheet* aSheet) {
   RemoveSheetOfType(aType, aSheet);
   AppendSheetOfType(aType, aSheet);
 
-  if (mRawSet) {
-    // Maintain a mirrored list of sheets on the servo side.
-    // Servo will remove aSheet from its original position as part of the call
-    // to Servo_StyleSet_AppendStyleSheet.
-    Servo_StyleSet_AppendStyleSheet(mRawSet.get(), aSheet);
-    SetStylistStyleSheetsDirty();
-  }
+  // Maintain a mirrored list of sheets on the servo side.
+  // Servo will remove aSheet from its original position as part of the call
+  // to Servo_StyleSet_AppendStyleSheet.
+  Servo_StyleSet_AppendStyleSheet(mRawSet.get(), aSheet);
+  SetStylistStyleSheetsDirty();
 
   if (mStyleRuleMap) {
     mStyleRuleMap->SheetAdded(*aSheet);
@@ -650,11 +619,10 @@ nsresult ServoStyleSet::RemoveStyleSheet(SheetType aType, StyleSheet* aSheet) {
   MOZ_ASSERT(IsCSSSheetType(aType));
 
   RemoveSheetOfType(aType, aSheet);
-  if (mRawSet) {
-    // Maintain a mirrored list of sheets on the servo side.
-    Servo_StyleSet_RemoveStyleSheet(mRawSet.get(), aSheet);
-    SetStylistStyleSheetsDirty();
-  }
+
+  // Maintain a mirrored list of sheets on the servo side.
+  Servo_StyleSet_RemoveStyleSheet(mRawSet.get(), aSheet);
+  SetStylistStyleSheetsDirty();
 
   if (mStyleRuleMap) {
     mStyleRuleMap->SheetRemoved(*aSheet);
@@ -675,20 +643,16 @@ nsresult ServoStyleSet::ReplaceSheets(
   // Remove all the existing sheets first.
   for (const auto& sheet : mSheets[aType]) {
     sheet->DropStyleSet(this);
-    if (mRawSet) {
-      Servo_StyleSet_RemoveStyleSheet(mRawSet.get(), sheet);
-    }
+    Servo_StyleSet_RemoveStyleSheet(mRawSet.get(), sheet);
   }
   mSheets[aType].Clear();
 
   // Add in all the new sheets.
   for (auto& sheet : aNewSheets) {
     AppendSheetOfType(aType, sheet);
-    if (mRawSet) {
-      MOZ_ASSERT(sheet->RawContents(),
-                 "Raw sheet should be in place before replacement.");
-      Servo_StyleSet_AppendStyleSheet(mRawSet.get(), sheet);
-    }
+    MOZ_ASSERT(sheet->RawContents(),
+               "Raw sheet should be in place before replacement.");
+    Servo_StyleSet_AppendStyleSheet(mRawSet.get(), sheet);
   }
 
   // Just don't bother calling SheetRemoved / SheetAdded, and recreate the rule
@@ -714,12 +678,10 @@ nsresult ServoStyleSet::InsertStyleSheetBefore(SheetType aType,
   RemoveSheetOfType(aType, aNewSheet);
   InsertSheetOfType(aType, aNewSheet, aReferenceSheet);
 
-  if (mRawSet) {
-    // Maintain a mirrored list of sheets on the servo side.
-    Servo_StyleSet_InsertStyleSheetBefore(mRawSet.get(), aNewSheet,
-                                          aReferenceSheet);
-    SetStylistStyleSheetsDirty();
-  }
+  // Maintain a mirrored list of sheets on the servo side.
+  Servo_StyleSet_InsertStyleSheetBefore(mRawSet.get(), aNewSheet,
+                                        aReferenceSheet);
+  SetStylistStyleSheetsDirty();
 
   if (mStyleRuleMap) {
     mStyleRuleMap->SheetAdded(*aNewSheet);
@@ -772,20 +734,16 @@ nsresult ServoStyleSet::AddDocStyleSheet(StyleSheet* aSheet,
     StyleSheet* beforeSheet = mSheets[SheetType::Doc][index];
     InsertSheetOfType(SheetType::Doc, aSheet, beforeSheet);
 
-    if (mRawSet) {
-      // Maintain a mirrored list of sheets on the servo side.
-      Servo_StyleSet_InsertStyleSheetBefore(mRawSet.get(), aSheet, beforeSheet);
-      SetStylistStyleSheetsDirty();
-    }
+    // Maintain a mirrored list of sheets on the servo side.
+    Servo_StyleSet_InsertStyleSheetBefore(mRawSet.get(), aSheet, beforeSheet);
+    SetStylistStyleSheetsDirty();
   } else {
     // This case is append.
     AppendSheetOfType(SheetType::Doc, aSheet);
 
-    if (mRawSet) {
-      // Maintain a mirrored list of sheets on the servo side.
-      Servo_StyleSet_AppendStyleSheet(mRawSet.get(), aSheet);
-      SetStylistStyleSheetsDirty();
-    }
+    // Maintain a mirrored list of sheets on the servo side.
+    Servo_StyleSet_AppendStyleSheet(mRawSet.get(), aSheet);
+    SetStylistStyleSheetsDirty();
   }
 
   if (mStyleRuleMap) {
@@ -970,10 +928,6 @@ void ServoStyleSet::StyleNewSubtree(Element* aRoot) {
 }
 
 void ServoStyleSet::MarkOriginsDirty(OriginFlags aChangedOrigins) {
-  if (MOZ_UNLIKELY(!mRawSet)) {
-    return;
-  }
-
   SetStylistStyleSheetsDirty();
   Servo_StyleSet_NoteStyleSheetsChanged(mRawSet.get(), aChangedOrigins);
 }
diff --git a/layout/style/ServoStyleSet.h b/layout/style/ServoStyleSet.h
index 206d44f52254..ead9343c5643 100644
--- a/layout/style/ServoStyleSet.h
+++ b/layout/style/ServoStyleSet.h
@@ -95,12 +95,11 @@ class ServoStyleSet {
 
   static ServoStyleSet* Current() { return sInServoTraversal; }
 
-  ServoStyleSet();
+  explicit ServoStyleSet(dom::Document&);
   ~ServoStyleSet();
 
-  void Init(nsPresContext* aPresContext);
-  void BeginShutdown() {}
-  void Shutdown();
+  void ShellAttachedToDocument();
+  void ShellDetachedFromDocument();
 
   // Called when a rules in a stylesheet in this set, or a child sheet of that,
   // are mutated from CSSOM.
@@ -505,10 +504,8 @@ class ServoStyleSet {
 
   void RemoveSheetOfType(SheetType aType, StyleSheet* aSheet);
 
-  // The owner document of this style set. Null if this is an XBL style set.
-  //
-  // TODO(emilio): This should become a DocumentOrShadowRoot, and be owned by it
-  // directly instead of the shell, eventually.
+  // The owner document of this style set. Never null, and always outlives the
+  // StyleSet.
   dom::Document* mDocument;
 
   const nsPresContext* GetPresContext() const {
diff --git a/servo/ports/geckolib/glue.rs b/servo/ports/geckolib/glue.rs
index 337fb32df7fa..053ca429f5fd 100644
--- a/servo/ports/geckolib/glue.rs
+++ b/servo/ports/geckolib/glue.rs
@@ -3517,14 +3517,8 @@ pub extern "C" fn Servo_ComputedValues_GetStyleRuleList(
     }))
 }
 
-/// See the comment in `Device` to see why it's ok to pass an owned reference to
-/// the pres context (hint: the context outlives the StyleSet, that holds the
-/// device alive).
 #[no_mangle]
-pub extern "C" fn Servo_StyleSet_Init(
-    pres_context: &structs::nsPresContext,
-) -> *mut RawServoStyleSet {
-    let doc = pres_context.mDocument.mRawPtr;
+pub extern "C" fn Servo_StyleSet_Init(doc: &structs::Document) -> *mut RawServoStyleSet {
     let data = Box::new(PerDocumentStyleData::new(doc));
     Box::into_raw(data) as *mut RawServoStyleSet
 }