--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -3746,26 +3746,17 @@ nsIDocument::SetHeaderData(nsAtom* aHead
     }
   }
 
   if (aHeaderField == nsGkAtoms::headerContentLanguage) {
     CopyUTF16toUTF8(aData, mContentLanguage);
   }
 
   if (aHeaderField == nsGkAtoms::headerDefaultStyle) {
-    // Only mess with our stylesheets if we don't have a lastStyleSheetSet, per
-    // spec.
-    if (DOMStringIsNull(mLastStyleSheetSet)) {
-      // Calling EnableStyleSheetsForSetInternal, not SetSelectedStyleSheetSet,
-      // per spec.  The idea here is that we're changing our preferred set and
-      // that shouldn't change the value of lastStyleSheetSet.  Also, we're
-      // using the Internal version so we can update the CSSLoader and not have
-      // to worry about null strings.
-      EnableStyleSheetsForSetInternal(aData, true);
-    }
+    SetPreferredStyleSheetSet(aData);
   }
 
   if (aHeaderField == nsGkAtoms::refresh) {
     // We get into this code before we have a script global yet, so get to
     // our container via mDocumentContainer.
     nsCOMPtr<nsIRefreshURI> refresher(mDocumentContainer);
     if (refresher) {
       // Note: using mDocumentURI instead of mBaseURI here, for consistency
@@ -6046,25 +6037,29 @@ nsIDocument::SetSelectedStyleSheetSet(co
 
   // Must update mLastStyleSheetSet before doing anything else with stylesheets
   // or CSSLoaders.
   mLastStyleSheetSet = aSheetSet;
   EnableStyleSheetsForSetInternal(aSheetSet, true);
 }
 
 void
-nsIDocument::GetLastStyleSheetSet(nsAString& aSheetSet)
-{
-  aSheetSet = mLastStyleSheetSet;
-}
-
-void
-nsIDocument::GetPreferredStyleSheetSet(nsAString& aSheetSet)
-{
-  GetHeaderData(nsGkAtoms::headerDefaultStyle, aSheetSet);
+nsIDocument::SetPreferredStyleSheetSet(const nsAString& aSheetSet)
+{
+  mPreferredStyleSheetSet = aSheetSet;
+  // Only mess with our stylesheets if we don't have a lastStyleSheetSet, per
+  // spec.
+  if (DOMStringIsNull(mLastStyleSheetSet)) {
+    // Calling EnableStyleSheetsForSetInternal, not SetSelectedStyleSheetSet,
+    // per spec.  The idea here is that we're changing our preferred set and
+    // that shouldn't change the value of lastStyleSheetSet.  Also, we're
+    // using the Internal version so we can update the CSSLoader and not have
+    // to worry about null strings.
+    EnableStyleSheetsForSetInternal(aSheetSet, true);
+  }
 }
 
 DOMStringList*
 nsIDocument::StyleSheetSets()
 {
   if (!mStyleSheetSetList) {
     mStyleSheetSetList = new nsDOMStyleSheetSetList(this);
   }
@@ -6081,32 +6076,32 @@ nsIDocument::EnableStyleSheetsForSet(con
     // non-null) or to our preferredStyleSheetSet.  And this method doesn't
     // change either of those.
     EnableStyleSheetsForSetInternal(aSheetSet, false);
   }
 }
 
 void
 nsIDocument::EnableStyleSheetsForSetInternal(const nsAString& aSheetSet,
-                                            bool aUpdateCSSLoader)
+                                             bool aUpdateCSSLoader)
 {
   BeginUpdate(UPDATE_STYLE);
   size_t count = SheetCount();
   nsAutoString title;
   for (size_t index = 0; index < count; index++) {
     StyleSheet* sheet = SheetAt(index);
     NS_ASSERTION(sheet, "Null sheet in sheet list!");
 
     sheet->GetTitle(title);
     if (!title.IsEmpty()) {
       sheet->SetEnabled(title.Equals(aSheetSet));
     }
   }
   if (aUpdateCSSLoader) {
-    CSSLoader()->SetPreferredSheet(aSheetSet);
+    CSSLoader()->DocumentStyleSheetSetChanged();
   }
   EndUpdate(UPDATE_STYLE);
 }
 
 void
 nsIDocument::GetCharacterSet(nsAString& aCharacterSet) const
 {
   nsAutoCString charset;

--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -3259,18 +3259,31 @@ public:
   }
   mozilla::dom::VisibilityState VisibilityState() const
   {
     return mVisibilityState;
   }
 #endif
   void GetSelectedStyleSheetSet(nsAString& aSheetSet);
   void SetSelectedStyleSheetSet(const nsAString& aSheetSet);
-  void GetLastStyleSheetSet(nsAString& aSheetSet);
-  void GetPreferredStyleSheetSet(nsAString& aSheetSet);
+  void GetLastStyleSheetSet(nsAString& aSheetSet)
+  {
+    aSheetSet = mLastStyleSheetSet;
+  }
+  const nsString& GetCurrentStyleSheetSet() const
+  {
+    return mLastStyleSheetSet.IsEmpty()
+      ? mPreferredStyleSheetSet
+      : mLastStyleSheetSet;
+  }
+  void SetPreferredStyleSheetSet(const nsAString&);
+  void GetPreferredStyleSheetSet(nsAString& aSheetSet)
+  {
+    aSheetSet = mPreferredStyleSheetSet;
+  }
   mozilla::dom::DOMStringList* StyleSheetSets();
   void EnableStyleSheetsForSet(const nsAString& aSheetSet);
 
   /**
    * Retrieve the location of the caret position (DOM node and character
    * offset within that node), given a point.
    *
    * @param aX Horizontal point at which to determine the caret position, in
@@ -3751,16 +3764,18 @@ protected:
                                  nsAtom* aAtom, void* aData);
   static void* UseExistingNameString(nsINode* aRootNode, const nsString* aName);
 
   void MaybeResolveReadyForIdle();
 
   nsCString mReferrer;
   nsString mLastModified;
 
+  nsString mPreferredStyleSheetSet;
+
   nsCOMPtr<nsIURI> mDocumentURI;
   nsCOMPtr<nsIURI> mOriginalURI;
   nsCOMPtr<nsIURI> mChromeXHRDocURI;
   nsCOMPtr<nsIURI> mDocumentBaseURI;
   nsCOMPtr<nsIURI> mChromeXHRDocBaseURI;
 
   // A lazily-constructed URL data for style system to resolve URL value.
   RefPtr<mozilla::URLExtraData> mCachedURLData;

--- a/dom/xul/nsXULContentSink.cpp
+++ b/dom/xul/nsXULContentSink.cpp
@@ -274,39 +274,20 @@ XULContentSinkImpl::GetTarget()
 nsresult
 XULContentSinkImpl::Init(nsIDocument* aDocument,
                          nsXULPrototypeDocument* aPrototype)
 {
     NS_PRECONDITION(aDocument != nullptr, "null ptr");
     if (! aDocument)
         return NS_ERROR_NULL_POINTER;
 
-    nsresult rv;
-
     mDocument    = do_GetWeakReference(aDocument);
     mPrototype   = aPrototype;
 
     mDocumentURL = mPrototype->GetURI();
-
-    // XXX this presumes HTTP header info is already set in document
-    // XXX if it isn't we need to set it here...
-    // XXXbz not like GetHeaderData on the proto doc _does_ anything....
-    nsAutoString preferredStyle;
-    rv = mPrototype->GetHeaderData(nsGkAtoms::headerDefaultStyle,
-                                   preferredStyle);
-    if (NS_FAILED(rv)) return rv;
-
-    if (!preferredStyle.IsEmpty()) {
-        aDocument->SetHeaderData(nsGkAtoms::headerDefaultStyle,
-                                 preferredStyle);
-    }
-
-    // Set the right preferred style on the document's CSSLoader.
-    aDocument->CSSLoader()->SetPreferredSheet(preferredStyle);
-
     mNodeInfoManager = aPrototype->GetNodeInfoManager();
     if (! mNodeInfoManager)
         return NS_ERROR_UNEXPECTED;
 
     mState = eInProlog;
     return NS_OK;
 }
 

--- a/layout/style/Loader.cpp
+++ b/layout/style/Loader.cpp
@@ -390,21 +390,16 @@ Loader::Loader(DocGroup* aDocGroup)
   mDocGroup = aDocGroup;
 }
 
 Loader::Loader(nsIDocument* aDocument)
   : Loader()
 {
   mDocument = aDocument;
   MOZ_ASSERT(mDocument, "We should get a valid document from the caller!");
-
-  // We can just use the preferred set, since there are no sheets in the
-  // document yet (if there are, how did they get there? _we_ load the sheets!)
-  // and hence the selected set makes no sense at this time.
-  mDocument->GetPreferredStyleSheetSet(mPreferredSheet);
 }
 
 Loader::~Loader()
 {
   NS_ASSERTION(!mSheets || mSheets->mLoadingDatas.Count() == 0,
                "How did we get destroyed when there are loading data?");
   NS_ASSERTION(!mSheets || mSheets->mPendingDatas.Count() == 0,
                "How did we get destroyed when there are pending data?");
@@ -420,57 +415,45 @@ Loader::DropDocumentReference(void)
   // Flush out pending datas just so we don't leak by accident.  These
   // loads should short-circuit through the mDocument check in
   // LoadSheet and just end up in SheetComplete immediately
   if (mSheets) {
     StartDeferredLoads();
   }
 }
 
-nsresult
-Loader::SetPreferredSheet(const nsAString& aTitle)
+void
+Loader::DocumentStyleSheetSetChanged()
 {
-#ifdef DEBUG
-  if (mDocument) {
-    nsAutoString currentPreferred;
-    mDocument->GetLastStyleSheetSet(currentPreferred);
-    if (DOMStringIsNull(currentPreferred)) {
-      mDocument->GetPreferredStyleSheetSet(currentPreferred);
-    }
-    NS_ASSERTION(currentPreferred.Equals(aTitle),
-                 "Unexpected argument to SetPreferredSheet");
-  }
-#endif
-
-  mPreferredSheet = aTitle;
+  MOZ_ASSERT(mDocument, "Huh?");
 
   // start any pending alternates that aren't alternates anymore
-  if (mSheets) {
-    LoadDataArray arr(mSheets->mPendingDatas.Count());
-    for (auto iter = mSheets->mPendingDatas.Iter(); !iter.Done(); iter.Next()) {
-      SheetLoadData* data = iter.Data();
-      MOZ_ASSERT(data, "Must have a data");
+  if (!mSheets) {
+    return;
+  }
 
-      // Note that we don't want to affect what the selected style set is, so
-      // use true for aHasAlternateRel.
-      auto isAlternate = data->mLoader->IsAlternateSheet(data->mTitle, true);
-      if (isAlternate == IsAlternate::No) {
-        arr.AppendElement(data);
-        iter.Remove();
-      }
-    }
+  LoadDataArray arr(mSheets->mPendingDatas.Count());
+  for (auto iter = mSheets->mPendingDatas.Iter(); !iter.Done(); iter.Next()) {
+    SheetLoadData* data = iter.Data();
+    MOZ_ASSERT(data, "Must have a data");
 
-    mDatasToNotifyOn += arr.Length();
-    for (uint32_t i = 0; i < arr.Length(); ++i) {
-      --mDatasToNotifyOn;
-      LoadSheet(arr[i], eSheetNeedsParser, false);
+    // Note that we don't want to affect what the selected style set is, so
+    // use true for aHasAlternateRel.
+    auto isAlternate = data->mLoader->IsAlternateSheet(data->mTitle, true);
+    if (isAlternate == IsAlternate::No) {
+      arr.AppendElement(data);
+      iter.Remove();
     }
   }
 
-  return NS_OK;
+  mDatasToNotifyOn += arr.Length();
+  for (uint32_t i = 0; i < arr.Length(); ++i) {
+    --mDatasToNotifyOn;
+    LoadSheet(arr[i], eSheetNeedsParser, false);
+  }
 }
 
 static const char kCharsetSym[] = "@charset \"";
 
 static bool GetCharsetFromData(const char* aStyleSheetData,
                                uint32_t aDataLength,
                                nsACString& aCharset)
 {
@@ -847,26 +830,31 @@ Loader::IsAlternateSheet(const nsAString
   // that's a preferred sheet.
   //
   // FIXME(emilio): This should return false for Shadow DOM regardless of the
   // document.
   if (aTitle.IsEmpty()) {
     return IsAlternate::No;
   }
 
-  if (!aHasAlternateRel && mDocument && mPreferredSheet.IsEmpty()) {
-    // There's no preferred set yet, and we now have a sheet with a title.
-    // Make that be the preferred set.
-    mDocument->SetHeaderData(nsGkAtoms::headerDefaultStyle, aTitle);
-    // We're definitely not an alternate.
-    return IsAlternate::No;
-  }
+  if (mDocument) {
+    const nsString& currentSheetSet = mDocument->GetCurrentStyleSheetSet();
+    if (!aHasAlternateRel && currentSheetSet.IsEmpty()) {
+      // There's no preferred set yet, and we now have a sheet with a title.
+      // Make that be the preferred set.
+      // FIXME(emilio): This is kinda wild, can we do it somewhere else?
+      mDocument->SetPreferredStyleSheetSet(aTitle);
+      // We're definitely not an alternate. Also, beware that at this point
+      // currentSheetSet may dangle.
+      return IsAlternate::No;
+    }
 
-  if (aTitle.Equals(mPreferredSheet)) {
-    return IsAlternate::No;
+    if (aTitle.Equals(currentSheetSet)) {
+      return IsAlternate::No;
+    }
   }
 
   return IsAlternate::Yes;
 }
 
 nsresult
 Loader::ObsoleteSheet(nsIURI* aURI)
 {
@@ -2680,17 +2668,16 @@ Loader::SizeOfIncludingThis(mozilla::Mal
   // Measurement of the following members may be added later if DMD finds it is
   // worthwhile:
   // - mLoadingDatas: transient, and should be small
   // - mPendingDatas: transient, and should be small
   // - mPostedEvents: transient, and should be small
   //
   // The following members aren't measured:
   // - mDocument, because it's a weak backpointer
-  // - mPreferredSheet, because it can be a shared string
 
   return n;
 }
 
 void
 Loader::BlockOnload()
 {
   if (mDocument) {

--- a/layout/style/Loader.h
+++ b/layout/style/Loader.h
@@ -211,17 +211,20 @@ public:
   NS_INLINE_DECL_CYCLE_COLLECTING_NATIVE_REFCOUNTING(Loader)
   NS_DECL_CYCLE_COLLECTION_NATIVE_CLASS(Loader)
 
   void DropDocumentReference(); // notification that doc is going away
 
   void SetCompatibilityMode(nsCompatibility aCompatMode)
   { mCompatMode = aCompatMode; }
   nsCompatibility GetCompatibilityMode() { return mCompatMode; }
-  nsresult SetPreferredSheet(const nsAString& aTitle);
+
+  // TODO(emilio): Is the complexity of this method and carrying the titles
+  // around worth it? The alternate sheets will load anyhow eventually...
+  void DocumentStyleSheetSetChanged();
 
   // XXXbz sort out what the deal is with events!  When should they fire?
 
   /**
    * Load an inline style sheet.  If a successful result is returned and
    * result.WillNotify() is true, then aObserver is guaranteed to be notified
    * asynchronously once the sheet is marked complete.  If an error is
    * returned, or if result.WillNotify() is false, aObserver will not be
@@ -623,17 +626,16 @@ private:
 
   // Number of datas still waiting to be notified on if we're notifying on a
   // whole bunch at once (e.g. in one of the stop methods).  This is used to
   // make sure that HasPendingLoads() won't return false until we're notifying
   // on the last data we're working with.
   uint32_t          mDatasToNotifyOn;
 
   nsCompatibility   mCompatMode;
-  nsString          mPreferredSheet;  // title of preferred sheet
 
   bool              mEnabled; // is enabled to load new styles
 
   nsCOMPtr<nsIConsoleReportCollector> mReporter;
 
 #ifdef DEBUG
   bool              mSyncCallback;
 #endif