Add m-esr52 at 52.6.0

1 files changed, 2694 insertions, 0 deletions

diff --git a/layout/style/Loader.cpp b/layout/style/Loader.cpp
new file mode 100644
index 000000000..a1a0fcfd9
--- /dev/null
+++ b/layout/style/Loader.cpp
@@ -0,0 +1,2694 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: ft=cpp tw=78 sw=2 et ts=2
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * 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/.
+ *
+ * This Original Code has been modified by IBM Corporation.
+ * Modifications made by IBM described herein are Copyright (c)
+ * International Business Machines Corporation, 2000.  Modifications
+ * to Mozilla code or documentation identified per MPL Section 3.3
+ *
+ * Date             Modified by     Description of modification
+ * 04/20/2000       IBM Corp.      OS/2 VisualAge build.
+ */
+
+/* loading of CSS style sheets using the network APIs */
+
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/LoadInfo.h"
+#include "mozilla/MemoryReporting.h"
+
+#include "mozilla/css/Loader.h"
+#include "mozilla/StyleSheetInlines.h"
+#include "nsIRunnable.h"
+#include "nsIUnicharStreamLoader.h"
+#include "nsSyncLoadService.h"
+#include "nsCOMPtr.h"
+#include "nsString.h"
+#include "nsIContent.h"
+#include "nsIDocument.h"
+#include "nsIDOMNode.h"
+#include "nsIDOMDocument.h"
+#include "nsIURI.h"
+#include "nsNetUtil.h"
+#include "nsIProtocolHandler.h"
+#include "nsContentUtils.h"
+#include "nsIScriptSecurityManager.h"
+#include "nsContentPolicyUtils.h"
+#include "nsIHttpChannel.h"
+#include "nsIHttpChannelInternal.h"
+#include "nsIClassOfService.h"
+#include "nsIScriptError.h"
+#include "nsMimeTypes.h"
+#include "nsIStyleSheetLinkingElement.h"
+#include "nsICSSLoaderObserver.h"
+#include "nsCSSParser.h"
+#include "mozilla/css/ImportRule.h"
+#include "nsThreadUtils.h"
+#include "nsGkAtoms.h"
+#include "nsIThreadInternal.h"
+#include "nsINetworkPredictor.h"
+#include "mozilla/dom/ShadowRoot.h"
+#include "mozilla/dom/URL.h"
+#include "mozilla/AsyncEventDispatcher.h"
+#include "mozilla/StyleSheet.h"
+#include "mozilla/StyleSheetInlines.h"
+#include "mozilla/ConsoleReportCollector.h"
+
+#ifdef MOZ_XUL
+#include "nsXULPrototypeCache.h"
+#endif
+
+#include "nsIMediaList.h"
+#include "nsIDOMStyleSheet.h"
+#include "nsError.h"
+
+#include "nsIContentSecurityPolicy.h"
+#include "mozilla/dom/SRICheck.h"
+
+#include "mozilla/dom/EncodingUtils.h"
+using mozilla::dom::EncodingUtils;
+
+using namespace mozilla::dom;
+
+/**
+ * OVERALL ARCHITECTURE
+ *
+ * The CSS Loader gets requests to load various sorts of style sheets:
+ * inline style from <style> elements, linked style, @import-ed child
+ * sheets, non-document sheets.  The loader handles the following tasks:
+ * 1) Creation of the actual style sheet objects: CreateSheet()
+ * 2) setting of the right media, title, enabled state, etc on the
+ *    sheet: PrepareSheet()
+ * 3) Insertion of the sheet in the proper cascade order:
+ *    InsertSheetInDoc() and InsertChildSheet()
+ * 4) Load of the sheet: LoadSheet() including security checks
+ * 5) Parsing of the sheet: ParseSheet()
+ * 6) Cleanup: SheetComplete()
+ *
+ * The detailed documentation for these functions is found with the
+ * function implementations.
+ *
+ * The following helper object is used:
+ *    SheetLoadData -- a small class that is used to store all the
+ *                     information needed for the loading of a sheet;
+ *                     this class handles listening for the stream
+ *                     loader completion and also handles charset
+ *                     determination.
+ */
+
+namespace mozilla {
+namespace css {
+
+/*********************************************
+ * Data needed to properly load a stylesheet *
+ *********************************************/
+
+static_assert(eAuthorSheetFeatures == 0 &&
+              eUserSheetFeatures == 1 &&
+              eAgentSheetFeatures == 2,
+              "sheet parsing mode constants won't fit "
+              "in SheetLoadData::mParsingMode");
+
+class SheetLoadData final : public nsIRunnable,
+                            public nsIUnicharStreamLoaderObserver,
+                            public nsIThreadObserver
+{
+protected:
+  virtual ~SheetLoadData(void);
+
+public:
+  // Data for loading a sheet linked from a document
+  SheetLoadData(Loader* aLoader,
+                const nsSubstring& aTitle,
+                nsIURI* aURI,
+                StyleSheet* aSheet,
+                nsIStyleSheetLinkingElement* aOwningElement,
+                bool aIsAlternate,
+                nsICSSLoaderObserver* aObserver,
+                nsIPrincipal* aLoaderPrincipal,
+                nsINode* aRequestingNode);
+
+  // Data for loading a sheet linked from an @import rule
+  SheetLoadData(Loader* aLoader,
+                nsIURI* aURI,
+                StyleSheet* aSheet,
+                SheetLoadData* aParentData,
+                nsICSSLoaderObserver* aObserver,
+                nsIPrincipal* aLoaderPrincipal,
+                nsINode* aRequestingNode);
+
+  // Data for loading a non-document sheet
+  SheetLoadData(Loader* aLoader,
+                nsIURI* aURI,
+                StyleSheet* aSheet,
+                bool aSyncLoad,
+                bool aUseSystemPrincipal,
+                const nsCString& aCharset,
+                nsICSSLoaderObserver* aObserver,
+                nsIPrincipal* aLoaderPrincipal,
+                nsINode* aRequestingNode);
+
+  already_AddRefed<nsIURI> GetReferrerURI();
+
+  void ScheduleLoadEventIfNeeded(nsresult aStatus);
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIRUNNABLE
+  NS_DECL_NSITHREADOBSERVER
+  NS_DECL_NSIUNICHARSTREAMLOADEROBSERVER
+
+  // Hold a ref to the CSSLoader so we can call back to it to let it
+  // know the load finished
+  RefPtr<Loader>           mLoader;
+
+  // Title needed to pull datas out of the pending datas table when
+  // the preferred title is changed
+  nsString                   mTitle;
+
+  // Charset we decided to use for the sheet
+  nsCString                  mCharset;
+
+  // URI we're loading.  Null for inline sheets
+  nsCOMPtr<nsIURI>           mURI;
+
+  // Should be 1 for non-inline sheets.
+  uint32_t                   mLineNumber;
+
+  // The sheet we're loading data for
+  RefPtr<StyleSheet> mSheet;
+
+  // Linked list of datas for the same URI as us
+  SheetLoadData*             mNext;  // strong ref
+
+  // Load data for the sheet that @import-ed us if we were @import-ed
+  // during the parse
+  RefPtr<SheetLoadData>    mParentData;
+
+  // Number of sheets we @import-ed that are still loading
+  uint32_t                   mPendingChildren;
+
+  // mSyncLoad is true when the load needs to be synchronous -- right
+  // now only for LoadSheetSync and children of sync loads.
+  bool                       mSyncLoad : 1;
+
+  // mIsNonDocumentSheet is true if the load was triggered by LoadSheetSync or
+  // LoadSheet or an @import from such a sheet.  Non-document sheet loads can
+  // proceed even if we have no document.
+  bool                       mIsNonDocumentSheet : 1;
+
+  // mIsLoading is true from the moment we are placed in the loader's
+  // "loading datas" table (right after the async channel is opened)
+  // to the moment we are removed from said table (due to the load
+  // completing or being cancelled).
+  bool                       mIsLoading : 1;
+
+  // mIsCancelled is set to true when a sheet load is stopped by
+  // Stop() or StopLoadingSheet() (which was removed in Bug 556446).
+  // SheetLoadData::OnStreamComplete() checks this to avoid parsing
+  // sheets that have been cancelled and such.
+  bool                       mIsCancelled : 1;
+
+  // mMustNotify is true if the load data is being loaded async and
+  // the original function call that started the load has returned.
+  // This applies only to observer notifications; load/error events
+  // are fired for any SheetLoadData that has a non-null
+  // mOwningElement.
+  bool                       mMustNotify : 1;
+
+  // mWasAlternate is true if the sheet was an alternate when the load data was
+  // created.
+  bool                       mWasAlternate : 1;
+
+  // mUseSystemPrincipal is true if the system principal should be used for
+  // this sheet, no matter what the channel principal is.  Only true for sync
+  // loads.
+  bool                       mUseSystemPrincipal : 1;
+
+  // If true, this SheetLoadData is being used as a way to handle
+  // async observer notification for an already-complete sheet.
+  bool                       mSheetAlreadyComplete : 1;
+
+  // This is the element that imported the sheet.  Needed to get the
+  // charset set on it and to fire load/error events.
+  nsCOMPtr<nsIStyleSheetLinkingElement> mOwningElement;
+
+  // The observer that wishes to be notified of load completion
+  nsCOMPtr<nsICSSLoaderObserver>        mObserver;
+
+  // The principal that identifies who started loading us.
+  nsCOMPtr<nsIPrincipal>                mLoaderPrincipal;
+
+  // The node that identifies who started loading us.
+  nsCOMPtr<nsINode>                     mRequestingNode;
+
+  // The charset to use if the transport and sheet don't indicate one.
+  // May be empty.  Must be empty if mOwningElement is non-null.
+  nsCString                             mCharsetHint;
+
+  // The status our load ended up with; this determines whether we
+  // should fire error events or load events.  This gets initialized
+  // by ScheduleLoadEventIfNeeded, and is only used after that has
+  // been called.
+  MOZ_INIT_OUTSIDE_CTOR nsresult        mStatus;
+
+private:
+  void FireLoadEvent(nsIThreadInternal* aThread);
+};
+
+#include "mozilla/Logging.h"
+
+static mozilla::LazyLogModule sCssLoaderLog("nsCSSLoader");
+
+static mozilla::LazyLogModule gSriPRLog("SRI");
+
+#define LOG_ERROR(args) MOZ_LOG(sCssLoaderLog, mozilla::LogLevel::Error, args)
+#define LOG_WARN(args) MOZ_LOG(sCssLoaderLog, mozilla::LogLevel::Warning, args)
+#define LOG_DEBUG(args) MOZ_LOG(sCssLoaderLog, mozilla::LogLevel::Debug, args)
+#define LOG(args) LOG_DEBUG(args)
+
+#define LOG_ERROR_ENABLED() MOZ_LOG_TEST(sCssLoaderLog, mozilla::LogLevel::Error)
+#define LOG_WARN_ENABLED() MOZ_LOG_TEST(sCssLoaderLog, mozilla::LogLevel::Warning)
+#define LOG_DEBUG_ENABLED() MOZ_LOG_TEST(sCssLoaderLog, mozilla::LogLevel::Debug)
+#define LOG_ENABLED() LOG_DEBUG_ENABLED()
+
+#define LOG_URI(format, uri)                        \
+  PR_BEGIN_MACRO                                    \
+    NS_ASSERTION(uri, "Logging null uri");          \
+    if (LOG_ENABLED()) {                            \
+      LOG((format, uri->GetSpecOrDefault().get())); \
+    }                                               \
+  PR_END_MACRO
+
+// And some convenience strings...
+static const char* const gStateStrings[] = {
+  "eSheetStateUnknown",
+  "eSheetNeedsParser",
+  "eSheetPending",
+  "eSheetLoading",
+  "eSheetComplete"
+};
+
+/********************************
+ * SheetLoadData implementation *
+ ********************************/
+NS_IMPL_ISUPPORTS(SheetLoadData, nsIUnicharStreamLoaderObserver, nsIRunnable,
+                  nsIThreadObserver)
+
+SheetLoadData::SheetLoadData(Loader* aLoader,
+                             const nsSubstring& aTitle,
+                             nsIURI* aURI,
+                             StyleSheet* aSheet,
+                             nsIStyleSheetLinkingElement* aOwningElement,
+                             bool aIsAlternate,
+                             nsICSSLoaderObserver* aObserver,
+                             nsIPrincipal* aLoaderPrincipal,
+                             nsINode* aRequestingNode)
+  : mLoader(aLoader),
+    mTitle(aTitle),
+    mURI(aURI),
+    mLineNumber(1),
+    mSheet(aSheet),
+    mNext(nullptr),
+    mPendingChildren(0),
+    mSyncLoad(false),
+    mIsNonDocumentSheet(false),
+    mIsLoading(false),
+    mIsCancelled(false),
+    mMustNotify(false),
+    mWasAlternate(aIsAlternate),
+    mUseSystemPrincipal(false),
+    mSheetAlreadyComplete(false),
+    mOwningElement(aOwningElement),
+    mObserver(aObserver),
+    mLoaderPrincipal(aLoaderPrincipal),
+    mRequestingNode(aRequestingNode)
+{
+  NS_PRECONDITION(mLoader, "Must have a loader!");
+}
+
+SheetLoadData::SheetLoadData(Loader* aLoader,
+                             nsIURI* aURI,
+                             StyleSheet* aSheet,
+                             SheetLoadData* aParentData,
+                             nsICSSLoaderObserver* aObserver,
+                             nsIPrincipal* aLoaderPrincipal,
+                             nsINode* aRequestingNode)
+  : mLoader(aLoader),
+    mURI(aURI),
+    mLineNumber(1),
+    mSheet(aSheet),
+    mNext(nullptr),
+    mParentData(aParentData),
+    mPendingChildren(0),
+    mSyncLoad(false),
+    mIsNonDocumentSheet(false),
+    mIsLoading(false),
+    mIsCancelled(false),
+    mMustNotify(false),
+    mWasAlternate(false),
+    mUseSystemPrincipal(false),
+    mSheetAlreadyComplete(false),
+    mOwningElement(nullptr),
+    mObserver(aObserver),
+    mLoaderPrincipal(aLoaderPrincipal),
+    mRequestingNode(aRequestingNode)
+{
+  NS_PRECONDITION(mLoader, "Must have a loader!");
+  if (mParentData) {
+    mSyncLoad = mParentData->mSyncLoad;
+    mIsNonDocumentSheet = mParentData->mIsNonDocumentSheet;
+    mUseSystemPrincipal = mParentData->mUseSystemPrincipal;
+    ++(mParentData->mPendingChildren);
+  }
+
+  NS_POSTCONDITION(!mUseSystemPrincipal || mSyncLoad,
+                   "Shouldn't use system principal for async loads");
+}
+
+SheetLoadData::SheetLoadData(Loader* aLoader,
+                             nsIURI* aURI,
+                             StyleSheet* aSheet,
+                             bool aSyncLoad,
+                             bool aUseSystemPrincipal,
+                             const nsCString& aCharset,
+                             nsICSSLoaderObserver* aObserver,
+                             nsIPrincipal* aLoaderPrincipal,
+                             nsINode* aRequestingNode)
+  : mLoader(aLoader),
+    mURI(aURI),
+    mLineNumber(1),
+    mSheet(aSheet),
+    mNext(nullptr),
+    mPendingChildren(0),
+    mSyncLoad(aSyncLoad),
+    mIsNonDocumentSheet(true),
+    mIsLoading(false),
+    mIsCancelled(false),
+    mMustNotify(false),
+    mWasAlternate(false),
+    mUseSystemPrincipal(aUseSystemPrincipal),
+    mSheetAlreadyComplete(false),
+    mOwningElement(nullptr),
+    mObserver(aObserver),
+    mLoaderPrincipal(aLoaderPrincipal),
+    mRequestingNode(aRequestingNode),
+    mCharsetHint(aCharset)
+{
+  NS_PRECONDITION(mLoader, "Must have a loader!");
+  NS_POSTCONDITION(!mUseSystemPrincipal || mSyncLoad,
+                   "Shouldn't use system principal for async loads");
+}
+
+SheetLoadData::~SheetLoadData()
+{
+  NS_CSS_NS_RELEASE_LIST_MEMBER(SheetLoadData, this, mNext);
+}
+
+NS_IMETHODIMP
+SheetLoadData::Run()
+{
+  mLoader->HandleLoadEvent(this);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SheetLoadData::OnDispatchedEvent(nsIThreadInternal* aThread)
+{
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SheetLoadData::OnProcessNextEvent(nsIThreadInternal* aThread,
+                                  bool aMayWait)
+{
+  // XXXkhuey this is insane!
+  // We want to fire our load even before or after event processing,
+  // whichever comes first.
+  FireLoadEvent(aThread);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+SheetLoadData::AfterProcessNextEvent(nsIThreadInternal* aThread,
+                                     bool aEventWasProcessed)
+{
+  // XXXkhuey this too!
+  // We want to fire our load even before or after event processing,
+  // whichever comes first.
+  FireLoadEvent(aThread);
+  return NS_OK;
+}
+
+void
+SheetLoadData::FireLoadEvent(nsIThreadInternal* aThread)
+{
+
+  // First remove ourselves as a thread observer.  But we need to keep
+  // ourselves alive while doing that!
+  RefPtr<SheetLoadData> kungFuDeathGrip(this);
+  aThread->RemoveObserver(this);
+
+  // Now fire the event
+  nsCOMPtr<nsINode> node = do_QueryInterface(mOwningElement);
+  NS_ASSERTION(node, "How did that happen???");
+
+  nsContentUtils::DispatchTrustedEvent(node->OwnerDoc(),
+                                       node,
+                                       NS_SUCCEEDED(mStatus) ?
+                                         NS_LITERAL_STRING("load") :
+                                         NS_LITERAL_STRING("error"),
+                                       false, false);
+
+  // And unblock onload
+  if (mLoader->mDocument) {
+    mLoader->mDocument->UnblockOnload(true);
+  }
+}
+
+void
+SheetLoadData::ScheduleLoadEventIfNeeded(nsresult aStatus)
+{
+  if (!mOwningElement) {
+    return;
+  }
+
+  mStatus = aStatus;
+
+  nsCOMPtr<nsIThread> thread = do_GetMainThread();
+  nsCOMPtr<nsIThreadInternal> internalThread = do_QueryInterface(thread);
+  if (NS_SUCCEEDED(internalThread->AddObserver(this))) {
+    // Make sure to block onload here
+    if (mLoader->mDocument) {
+      mLoader->mDocument->BlockOnload();
+    }
+  }
+}
+
+/*********************
+ * Style sheet reuse *
+ *********************/
+
+bool
+LoaderReusableStyleSheets::FindReusableStyleSheet(nsIURI* aURL,
+                                                  RefPtr<CSSStyleSheet>& aResult)
+{
+  MOZ_ASSERT(aURL);
+  for (size_t i = mReusableSheets.Length(); i > 0; --i) {
+    size_t index = i - 1;
+    bool sameURI;
+    MOZ_ASSERT(mReusableSheets[index]->GetOriginalURI());
+    nsresult rv = aURL->Equals(mReusableSheets[index]->GetOriginalURI(),
+                               &sameURI);
+    if (!NS_FAILED(rv) && sameURI) {
+      aResult = mReusableSheets[index];
+      mReusableSheets.RemoveElementAt(index);
+      return true;
+    }
+  }
+  return false;
+}
+
+/*************************
+ * Loader Implementation *
+ *************************/
+
+Loader::Loader(StyleBackendType aType)
+  : mDocument(nullptr)
+  , mDatasToNotifyOn(0)
+  , mCompatMode(eCompatibility_FullStandards)
+  , mStyleBackendType(Some(aType))
+  , mEnabled(true)
+  , mReporter(new ConsoleReportCollector())
+#ifdef DEBUG
+  , mSyncCallback(false)
+#endif
+{
+}
+
+Loader::Loader(nsIDocument* aDocument)
+  : mDocument(aDocument)
+  , mDatasToNotifyOn(0)
+  , mCompatMode(eCompatibility_FullStandards)
+  , mEnabled(true)
+  , mReporter(new ConsoleReportCollector())
+#ifdef DEBUG
+  , mSyncCallback(false)
+#endif
+{
+  // 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.
+  nsCOMPtr<nsIDOMDocument> domDoc = do_QueryInterface(mDocument);
+  if (domDoc) {
+    domDoc->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?");
+  // Note: no real need to revoke our stylesheet loaded events -- they
+  // hold strong references to us, so if we're going away that means
+  // they're all done.
+}
+
+void
+Loader::DropDocumentReference(void)
+{
+  mDocument = nullptr;
+  // 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) {
+    StartAlternateLoads();
+  }
+}
+
+nsresult
+Loader::SetPreferredSheet(const nsAString& aTitle)
+{
+#ifdef DEBUG
+  nsCOMPtr<nsIDOMDocument> doc = do_QueryInterface(mDocument);
+  if (doc) {
+    nsAutoString currentPreferred;
+    doc->GetLastStyleSheetSet(currentPreferred);
+    if (DOMStringIsNull(currentPreferred)) {
+      doc->GetPreferredStyleSheetSet(currentPreferred);
+    }
+    NS_ASSERTION(currentPreferred.Equals(aTitle),
+                 "Unexpected argument to SetPreferredSheet");
+  }
+#endif
+
+  mPreferredSheet = aTitle;
+
+  // 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");
+
+      // Note that we don't want to affect what the selected style set is, so
+      // use true for aHasAlternateRel.
+      if (!data->mLoader->IsAlternate(data->mTitle, true)) {
+        arr.AppendElement(data);
+        iter.Remove();
+      }
+    }
+
+    mDatasToNotifyOn += arr.Length();
+    for (uint32_t i = 0; i < arr.Length(); ++i) {
+      --mDatasToNotifyOn;
+      LoadSheet(arr[i], eSheetNeedsParser, false);
+    }
+  }
+
+  return NS_OK;
+}
+
+static const char kCharsetSym[] = "@charset \"";
+
+static bool GetCharsetFromData(const char* aStyleSheetData,
+                               uint32_t aDataLength,
+                               nsACString& aCharset)
+{
+  aCharset.Truncate();
+  if (aDataLength <= sizeof(kCharsetSym) - 1)
+    return false;
+
+  if (strncmp(aStyleSheetData,
+              kCharsetSym,
+              sizeof(kCharsetSym) - 1)) {
+    return false;
+  }
+
+  for (uint32_t i = sizeof(kCharsetSym) - 1; i < aDataLength; ++i) {
+    char c = aStyleSheetData[i];
+    if (c == '"') {
+      ++i;
+      if (i < aDataLength && aStyleSheetData[i] == ';') {
+        return true;
+      }
+      // fail
+      break;
+    }
+    aCharset.Append(c);
+  }
+
+  // Did not see end quote or semicolon
+  aCharset.Truncate();
+  return false;
+}
+
+NS_IMETHODIMP
+SheetLoadData::OnDetermineCharset(nsIUnicharStreamLoader* aLoader,
+                                  nsISupports* aContext,
+                                  nsACString const& aSegment,
+                                  nsACString& aCharset)
+{
+  NS_PRECONDITION(!mOwningElement || mCharsetHint.IsEmpty(),
+                  "Can't have element _and_ charset hint");
+
+  LOG_URI("SheetLoadData::OnDetermineCharset for '%s'", mURI);
+
+  // The precedence is (per CSS3 Syntax 2012-11-08 ED):
+  // BOM
+  // Channel
+  // @charset rule
+  // charset attribute on the referrer
+  // encoding of the referrer
+  // UTF-8
+
+  aCharset.Truncate();
+
+  if (nsContentUtils::CheckForBOM((const unsigned char*)aSegment.BeginReading(),
+                                  aSegment.Length(),
+                                  aCharset)) {
+    // aCharset is now either "UTF-16BE", "UTF-16BE" or "UTF-8"
+    // which will swallow the BOM.
+    mCharset.Assign(aCharset);
+    LOG(("  Setting from BOM to: %s", PromiseFlatCString(aCharset).get()));
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsIChannel> channel;
+  nsAutoCString specified;
+  aLoader->GetChannel(getter_AddRefs(channel));
+  if (channel) {
+    channel->GetContentCharset(specified);
+    if (EncodingUtils::FindEncodingForLabel(specified, aCharset)) {
+      mCharset.Assign(aCharset);
+      LOG(("  Setting from HTTP to: %s", PromiseFlatCString(aCharset).get()));
+      return NS_OK;
+    }
+  }
+
+  if (GetCharsetFromData(aSegment.BeginReading(),
+                         aSegment.Length(),
+                         specified)) {
+    if (EncodingUtils::FindEncodingForLabel(specified, aCharset)) {
+      // FindEncodingForLabel currently never returns UTF-16LE but will
+      // probably change to never return UTF-16 instead, so check both here
+      // to avoid relying on the exact behavior.
+      if (aCharset.EqualsLiteral("UTF-16") ||
+          aCharset.EqualsLiteral("UTF-16BE") ||
+          aCharset.EqualsLiteral("UTF-16LE")) {
+        // Be consistent with HTML <meta> handling in face of impossibility.
+        // When the @charset rule itself evidently was not UTF-16-encoded,
+        // it saying UTF-16 has to be a lie.
+        aCharset.AssignLiteral("UTF-8");
+      }
+      mCharset.Assign(aCharset);
+      LOG(("  Setting from @charset rule to: %s",
+          PromiseFlatCString(aCharset).get()));
+      return NS_OK;
+    }
+  }
+
+  // Now try the charset on the <link> or processing instruction
+  // that loaded us
+  if (mOwningElement) {
+    nsAutoString specified16;
+    mOwningElement->GetCharset(specified16);
+    if (EncodingUtils::FindEncodingForLabel(specified16, aCharset)) {
+      mCharset.Assign(aCharset);
+      LOG(("  Setting from charset attribute to: %s",
+          PromiseFlatCString(aCharset).get()));
+      return NS_OK;
+    }
+  }
+
+  // In the preload case, the value of the charset attribute on <link> comes
+  // in via mCharsetHint instead.
+  if (EncodingUtils::FindEncodingForLabel(mCharsetHint, aCharset)) {
+    mCharset.Assign(aCharset);
+      LOG(("  Setting from charset attribute (preload case) to: %s",
+          PromiseFlatCString(aCharset).get()));
+    return NS_OK;
+  }
+
+  // Try charset from the parent stylesheet.
+  if (mParentData) {
+    aCharset = mParentData->mCharset;
+    if (!aCharset.IsEmpty()) {
+      mCharset.Assign(aCharset);
+      LOG(("  Setting from parent sheet to: %s",
+          PromiseFlatCString(aCharset).get()));
+      return NS_OK;
+    }
+  }
+
+  if (mLoader->mDocument) {
+    // no useful data on charset.  Try the document charset.
+    aCharset = mLoader->mDocument->GetDocumentCharacterSet();
+    MOZ_ASSERT(!aCharset.IsEmpty());
+    mCharset.Assign(aCharset);
+    LOG(("  Setting from document to: %s", PromiseFlatCString(aCharset).get()));
+    return NS_OK;
+  }
+
+  aCharset.AssignLiteral("UTF-8");
+  mCharset = aCharset;
+  LOG(("  Setting from default to: %s", PromiseFlatCString(aCharset).get()));
+  return NS_OK;
+}
+
+already_AddRefed<nsIURI>
+SheetLoadData::GetReferrerURI()
+{
+  nsCOMPtr<nsIURI> uri;
+  if (mParentData)
+    uri = mParentData->mSheet->GetSheetURI();
+  if (!uri && mLoader->mDocument)
+    uri = mLoader->mDocument->GetDocumentURI();
+  return uri.forget();
+}
+
+/*
+ * Here we need to check that the load did not give us an http error
+ * page and check the mimetype on the channel to make sure we're not
+ * loading non-text/css data in standards mode.
+ */
+NS_IMETHODIMP
+SheetLoadData::OnStreamComplete(nsIUnicharStreamLoader* aLoader,
+                                nsISupports* aContext,
+                                nsresult aStatus,
+                                const nsAString& aBuffer)
+{
+  LOG(("SheetLoadData::OnStreamComplete"));
+  NS_ASSERTION(!mLoader->mSyncCallback, "Synchronous callback from necko");
+
+  if (mIsCancelled) {
+    // Just return.  Don't call SheetComplete -- it's already been
+    // called and calling it again will lead to an extra NS_RELEASE on
+    // this data and a likely crash.
+    return NS_OK;
+  }
+
+  if (!mLoader->mDocument && !mIsNonDocumentSheet) {
+    // Sorry, we don't care about this load anymore
+    LOG_WARN(("  No document and not non-document sheet; dropping load"));
+    mLoader->SheetComplete(this, NS_BINDING_ABORTED);
+    return NS_OK;
+  }
+
+  if (NS_FAILED(aStatus)) {
+    LOG_WARN(("  Load failed: status 0x%x", aStatus));
+    // Handle sheet not loading error because source was a tracking URL.
+    // We make a note of this sheet node by including it in a dedicated
+    // array of blocked tracking nodes under its parent document.
+    //
+    // Multiple sheet load instances might be tied to this request,
+    // we annotate each one linked to a valid owning element (node).
+    if (aStatus == NS_ERROR_TRACKING_URI) {
+      nsIDocument* doc = mLoader->GetDocument();
+      if (doc) {
+        for (SheetLoadData* data = this; data; data = data->mNext) {
+          // mOwningElement may be null but AddBlockTrackingNode can cope
+          nsCOMPtr<nsIContent> content = do_QueryInterface(data->mOwningElement);
+          doc->AddBlockedTrackingNode(content);
+        }
+      }
+    }
+    mLoader->SheetComplete(this, aStatus);
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsIChannel> channel;
+  nsresult result = aLoader->GetChannel(getter_AddRefs(channel));
+  if (NS_FAILED(result)) {
+    LOG_WARN(("  No channel from loader"));
+    mLoader->SheetComplete(this, result);
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsIURI> originalURI;
+  channel->GetOriginalURI(getter_AddRefs(originalURI));
+
+  // If the channel's original URI is "chrome:", we want that, since
+  // the observer code in nsXULPrototypeCache depends on chrome stylesheets
+  // having a chrome URI.  (Whether or not chrome stylesheets come through
+  // this codepath seems nondeterministic.)
+  // Otherwise we want the potentially-HTTP-redirected URI.
+  nsCOMPtr<nsIURI> channelURI;
+  NS_GetFinalChannelURI(channel, getter_AddRefs(channelURI));
+
+  if (!channelURI || !originalURI) {
+    NS_ERROR("Someone just violated the nsIRequest contract");
+    LOG_WARN(("  Channel without a URI.  Bad!"));
+    mLoader->SheetComplete(this, NS_ERROR_UNEXPECTED);
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsIPrincipal> principal;
+  nsIScriptSecurityManager* secMan = nsContentUtils::GetSecurityManager();
+  result = NS_ERROR_NOT_AVAILABLE;
+  if (secMan) {  // Could be null if we already shut down
+    if (mUseSystemPrincipal) {
+      result = secMan->GetSystemPrincipal(getter_AddRefs(principal));
+    } else {
+      result = secMan->GetChannelResultPrincipal(channel, getter_AddRefs(principal));
+    }
+  }
+
+  if (NS_FAILED(result)) {
+    LOG_WARN(("  Couldn't get principal"));
+    mLoader->SheetComplete(this, result);
+    return NS_OK;
+  }
+
+  mSheet->SetPrincipal(principal);
+
+  // If it's an HTTP channel, we want to make sure this is not an
+  // error document we got.
+  nsCOMPtr<nsIHttpChannel> httpChannel(do_QueryInterface(channel));
+  if (httpChannel) {
+    bool requestSucceeded;
+    result = httpChannel->GetRequestSucceeded(&requestSucceeded);
+    if (NS_SUCCEEDED(result) && !requestSucceeded) {
+      LOG(("  Load returned an error page"));
+      mLoader->SheetComplete(this, NS_ERROR_NOT_AVAILABLE);
+      return NS_OK;
+    }
+  }
+
+  nsAutoCString contentType;
+  if (channel) {
+    channel->GetContentType(contentType);
+  }
+
+  // In standards mode, a style sheet must have one of these MIME
+  // types to be processed at all.  In quirks mode, we accept any
+  // MIME type, but only if the style sheet is same-origin with the
+  // requesting document or parent sheet.  See bug 524223.
+
+  bool validType = contentType.EqualsLiteral("text/css") ||
+    contentType.EqualsLiteral(UNKNOWN_CONTENT_TYPE) ||
+    contentType.IsEmpty();
+
+  if (!validType) {
+    const char *errorMessage;
+    uint32_t errorFlag;
+    bool sameOrigin = true;
+
+    if (mLoaderPrincipal) {
+      bool subsumed;
+      result = mLoaderPrincipal->Subsumes(principal, &subsumed);
+      if (NS_FAILED(result) || !subsumed) {
+        sameOrigin = false;
+      }
+    }
+
+    if (sameOrigin && mLoader->mCompatMode == eCompatibility_NavQuirks) {
+      errorMessage = "MimeNotCssWarn";
+      errorFlag = nsIScriptError::warningFlag;
+    } else {
+      errorMessage = "MimeNotCss";
+      errorFlag = nsIScriptError::errorFlag;
+    }
+
+    const nsAFlatString& specUTF16 =
+      NS_ConvertUTF8toUTF16(channelURI->GetSpecOrDefault());
+    const nsAFlatString& ctypeUTF16 = NS_ConvertASCIItoUTF16(contentType);
+    const char16_t *strings[] = { specUTF16.get(), ctypeUTF16.get() };
+
+    nsCOMPtr<nsIURI> referrer = GetReferrerURI();
+    nsContentUtils::ReportToConsole(errorFlag,
+                                    NS_LITERAL_CSTRING("CSS Loader"),
+                                    mLoader->mDocument,
+                                    nsContentUtils::eCSS_PROPERTIES,
+                                    errorMessage,
+                                    strings, ArrayLength(strings),
+                                    referrer);
+
+    if (errorFlag == nsIScriptError::errorFlag) {
+      LOG_WARN(("  Ignoring sheet with improper MIME type %s",
+                contentType.get()));
+      mLoader->SheetComplete(this, NS_ERROR_NOT_AVAILABLE);
+      return NS_OK;
+    }
+  }
+
+  SRIMetadata sriMetadata;
+  mSheet->GetIntegrity(sriMetadata);
+  if (sriMetadata.IsEmpty()) {
+    nsCOMPtr<nsILoadInfo> loadInfo = channel->GetLoadInfo();
+    if (loadInfo->GetEnforceSRI()) {
+      LOG(("  Load was blocked by SRI"));
+      MOZ_LOG(gSriPRLog, mozilla::LogLevel::Debug,
+              ("css::Loader::OnStreamComplete, required SRI not found"));
+      mLoader->SheetComplete(this, NS_ERROR_SRI_CORRUPT);
+      // log the failed load to web console
+      nsCOMPtr<nsIContentSecurityPolicy> csp;
+      loadInfo->LoadingPrincipal()->GetCsp(getter_AddRefs(csp));
+      nsAutoCString spec;
+      mLoader->mDocument->GetDocumentURI()->GetAsciiSpec(spec);
+      // line number unknown. mRequestingNode doesn't bear this info.
+      csp->LogViolationDetails(
+        nsIContentSecurityPolicy::VIOLATION_TYPE_REQUIRE_SRI_FOR_STYLE,
+        NS_ConvertUTF8toUTF16(spec), EmptyString(),
+        0, EmptyString(), EmptyString());
+      return NS_OK;
+    }
+  } else {
+    nsAutoCString sourceUri;
+    if (mLoader->mDocument && mLoader->mDocument->GetDocumentURI()) {
+      mLoader->mDocument->GetDocumentURI()->GetAsciiSpec(sourceUri);
+    }
+    nsresult rv = SRICheck::VerifyIntegrity(sriMetadata, aLoader, aBuffer,
+                                            sourceUri, mLoader->mReporter);
+    mLoader->mReporter->FlushConsoleReports(mLoader->mDocument);
+    if (NS_FAILED(rv)) {
+      LOG(("  Load was blocked by SRI"));
+      MOZ_LOG(gSriPRLog, mozilla::LogLevel::Debug,
+              ("css::Loader::OnStreamComplete, bad metadata"));
+      mLoader->SheetComplete(this, NS_ERROR_SRI_CORRUPT);
+      return NS_OK;
+    }
+  }
+
+  // Enough to set the URIs on mSheet, since any sibling datas we have share
+  // the same mInner as mSheet and will thus get the same URI.
+  mSheet->SetURIs(channelURI, originalURI, channelURI);
+
+  bool completed;
+  result = mLoader->ParseSheet(aBuffer, this, completed);
+  NS_ASSERTION(completed || !mSyncLoad, "sync load did not complete");
+  return result;
+}
+
+bool
+Loader::IsAlternate(const nsAString& aTitle, bool aHasAlternateRel)
+{
+  // A sheet is alternate if it has a nonempty title that doesn't match the
+  // currently selected style set.  But if there _is_ no currently selected
+  // style set, the sheet wasn't marked as an alternate explicitly, and aTitle
+  // is nonempty, we should select the style set corresponding to aTitle, since
+  // that's a preferred sheet.
+  if (aTitle.IsEmpty()) {
+    return false;
+  }
+
+  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 false;
+  }
+
+  return !aTitle.Equals(mPreferredSheet);
+}
+
+nsresult
+Loader::ObsoleteSheet(nsIURI* aURI)
+{
+  if (!mSheets) {
+    return NS_OK;
+  }
+  if (!aURI) {
+    return NS_ERROR_INVALID_ARG;
+  }
+  for (auto iter = mSheets->mCompleteSheets.Iter(); !iter.Done(); iter.Next()) {
+    nsIURI* sheetURI = iter.Key()->GetURI();
+    bool areEqual;
+    nsresult rv = sheetURI->Equals(aURI, &areEqual);
+    if (NS_SUCCEEDED(rv) && areEqual) {
+      iter.Remove();
+    }
+  }
+  return NS_OK;
+}
+
+nsresult
+Loader::CheckContentPolicy(nsIPrincipal* aSourcePrincipal,
+                          nsIURI* aTargetURI,
+                          nsISupports* aContext,
+                          bool aIsPreload)
+{
+  // When performing a system load (e.g. aUseSystemPrincipal = true)
+  // then aSourcePrincipal == null; don't consult content policies.
+  if (!aSourcePrincipal) {
+    return NS_OK;
+  }
+
+  nsContentPolicyType contentPolicyType =
+    aIsPreload ? nsIContentPolicy::TYPE_INTERNAL_STYLESHEET_PRELOAD
+               : nsIContentPolicy::TYPE_INTERNAL_STYLESHEET;
+
+  int16_t shouldLoad = nsIContentPolicy::ACCEPT;
+  nsresult rv = NS_CheckContentLoadPolicy(contentPolicyType,
+                                          aTargetURI,
+                                          aSourcePrincipal,
+                                          aContext,
+                                          NS_LITERAL_CSTRING("text/css"),
+                                          nullptr,  //extra param
+                                          &shouldLoad,
+                                          nsContentUtils::GetContentPolicy(),
+                                          nsContentUtils::GetSecurityManager());
+   if (NS_FAILED(rv) || NS_CP_REJECTED(shouldLoad)) {
+     return NS_ERROR_CONTENT_BLOCKED;
+   }
+   return NS_OK;
+}
+
+/**
+ * CreateSheet() creates a CSSStyleSheet object for the given URI,
+ * if any.  If there is no URI given, we just create a new style sheet
+ * object.  Otherwise, we check for an existing style sheet object for
+ * that uri in various caches and clone it if we find it.  Cloned
+ * sheets will have the title/media/enabled state of the sheet they
+ * are clones off; make sure to call PrepareSheet() on the result of
+ * CreateSheet().
+ */
+nsresult
+Loader::CreateSheet(nsIURI* aURI,
+                    nsIContent* aLinkingContent,
+                    nsIPrincipal* aLoaderPrincipal,
+                    css::SheetParsingMode aParsingMode,
+                    CORSMode aCORSMode,
+                    ReferrerPolicy aReferrerPolicy,
+                    const nsAString& aIntegrity,
+                    bool aSyncLoad,
+                    bool aHasAlternateRel,
+                    const nsAString& aTitle,
+                    StyleSheetState& aSheetState,
+                    bool *aIsAlternate,
+                    RefPtr<StyleSheet>* aSheet)
+{
+  LOG(("css::Loader::CreateSheet"));
+  NS_PRECONDITION(aSheet, "Null out param!");
+
+  if (!mSheets) {
+    mSheets = new Sheets();
+  }
+
+  *aSheet = nullptr;
+  aSheetState = eSheetStateUnknown;
+
+  // Check the alternate state before doing anything else, because it
+  // can mess with our hashtables.
+  *aIsAlternate = IsAlternate(aTitle, aHasAlternateRel);
+
+  // XXXheycam Cached sheets currently must be CSSStyleSheets.
+  if (aURI && GetStyleBackendType() == StyleBackendType::Gecko) {
+    aSheetState = eSheetComplete;
+    RefPtr<StyleSheet> sheet;
+
+    // First, the XUL cache
+#ifdef MOZ_XUL
+    if (IsChromeURI(aURI)) {
+      nsXULPrototypeCache* cache = nsXULPrototypeCache::GetInstance();
+      if (cache) {
+        if (cache->IsEnabled()) {
+          sheet = cache->GetStyleSheet(aURI);
+          LOG(("  From XUL cache: %p", sheet.get()));
+        }
+      }
+    }
+#endif
+
+    bool fromCompleteSheets = false;
+    if (!sheet) {
+      // Then our per-document complete sheets.
+      URIPrincipalReferrerPolicyAndCORSModeHashKey key(aURI, aLoaderPrincipal, aCORSMode, aReferrerPolicy);
+
+      StyleSheet* completeSheet = nullptr;
+      mSheets->mCompleteSheets.Get(&key, &completeSheet);
+      sheet = completeSheet;
+      LOG(("  From completed: %p", sheet.get()));
+
+      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->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->AsGecko()->IsModified()) {
+        LOG(("  Not cloning completed sheet %p because it's been modified",
+             sheet.get()));
+        sheet = nullptr;
+        fromCompleteSheets = false;
+      }
+    }
+
+    // Then loading sheets
+    if (!sheet && !aSyncLoad) {
+      aSheetState = eSheetLoading;
+      SheetLoadData* loadData = nullptr;
+      URIPrincipalReferrerPolicyAndCORSModeHashKey key(aURI, aLoaderPrincipal, aCORSMode, aReferrerPolicy);
+      mSheets->mLoadingDatas.Get(&key, &loadData);
+      if (loadData) {
+        sheet = loadData->mSheet;
+        LOG(("  From loading: %p", sheet.get()));
+
+#ifdef DEBUG
+        bool debugEqual;
+        NS_ASSERTION((!aLoaderPrincipal && !loadData->mLoaderPrincipal) ||
+                     (aLoaderPrincipal && loadData->mLoaderPrincipal &&
+                      NS_SUCCEEDED(aLoaderPrincipal->
+                                   Equals(loadData->mLoaderPrincipal,
+                                          &debugEqual)) && debugEqual),
+                     "Principals should be the same");
+#endif
+      }
+
+      // Then alternate sheets
+      if (!sheet) {
+        aSheetState = eSheetPending;
+        loadData = nullptr;
+        mSheets->mPendingDatas.Get(&key, &loadData);
+        if (loadData) {
+          sheet = loadData->mSheet;
+          LOG(("  From pending: %p", sheet.get()));
+
+#ifdef DEBUG
+          bool debugEqual;
+          NS_ASSERTION((!aLoaderPrincipal && !loadData->mLoaderPrincipal) ||
+                       (aLoaderPrincipal && loadData->mLoaderPrincipal &&
+                        NS_SUCCEEDED(aLoaderPrincipal->
+                                     Equals(loadData->mLoaderPrincipal,
+                                            &debugEqual)) && debugEqual),
+                       "Principals should be the same");
+#endif
+        }
+      }
+    }
+
+    if (sheet) {
+      // The sheet we have now should be either incomplete or unmodified
+      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->AsGecko()->IsComplete() ||
+                   aSheetState != eSheetComplete,
+                   "Sheet thinks it's not complete while we think it is");
+
+      RefPtr<CSSStyleSheet> clonedSheet =
+        sheet->AsGecko()->Clone(nullptr, nullptr, nullptr, nullptr);
+      *aSheet = Move(clonedSheet);
+      if (*aSheet && fromCompleteSheets &&
+          !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)->AsGecko()->IsComplete(),
+                     "Should only be caching complete sheets");
+        mSheets->mCompleteSheets.Put(&key, *aSheet);
+      }
+    }
+  }
+
+  if (!*aSheet) {
+    aSheetState = eSheetNeedsParser;
+    nsIURI *sheetURI;
+    nsCOMPtr<nsIURI> baseURI;
+    nsIURI* originalURI;
+    if (!aURI) {
+      // Inline style.  Use the document's base URL so that @import in
+      // the inline sheet picks up the right base.
+      NS_ASSERTION(aLinkingContent, "Inline stylesheet without linking content?");
+      baseURI = aLinkingContent->GetBaseURI();
+      sheetURI = aLinkingContent->OwnerDoc()->GetDocumentURI();
+      originalURI = nullptr;
+    } else {
+      baseURI = aURI;
+      sheetURI = aURI;
+      originalURI = aURI;
+    }
+
+    SRIMetadata sriMetadata;
+    if (!aIntegrity.IsEmpty()) {
+      MOZ_LOG(gSriPRLog, mozilla::LogLevel::Debug,
+              ("css::Loader::CreateSheet, integrity=%s",
+               NS_ConvertUTF16toUTF8(aIntegrity).get()));
+      nsAutoCString sourceUri;
+      if (mDocument && mDocument->GetDocumentURI()) {
+        mDocument->GetDocumentURI()->GetAsciiSpec(sourceUri);
+      }
+      SRICheck::IntegrityMetadata(aIntegrity, sourceUri, mReporter,
+                                  &sriMetadata);
+    }
+
+    if (GetStyleBackendType() == StyleBackendType::Gecko) {
+      *aSheet = new CSSStyleSheet(aParsingMode, aCORSMode, aReferrerPolicy, sriMetadata);
+    } else {
+      *aSheet = new ServoStyleSheet(aParsingMode, aCORSMode, aReferrerPolicy, sriMetadata);
+    }
+    (*aSheet)->SetURIs(sheetURI, originalURI, baseURI);
+  }
+
+  NS_ASSERTION(*aSheet, "We should have a sheet by now!");
+  NS_ASSERTION(aSheetState != eSheetStateUnknown, "Have to set a state!");
+  LOG(("  State: %s", gStateStrings[aSheetState]));
+
+  return NS_OK;
+}
+
+/**
+ * PrepareSheet() handles setting the media and title on the sheet, as
+ * well as setting the enabled state based on the title and whether
+ * the sheet had "alternate" in its rel.
+ */
+void
+Loader::PrepareSheet(StyleSheet* aSheet,
+                     const nsSubstring& aTitle,
+                     const nsSubstring& aMediaString,
+                     nsMediaList* aMediaList,
+                     Element* aScopeElement,
+                     bool isAlternate)
+{
+  NS_PRECONDITION(aSheet, "Must have a sheet!");
+
+  // XXXheycam Need to set media, title, etc. on ServoStyleSheets.
+  if (aSheet->IsServo()) {
+    NS_WARNING("stylo: should set metadata on ServoStyleSheets. See bug 1290209.");
+    return;
+  }
+
+  CSSStyleSheet* sheet = aSheet->AsGecko();
+
+  RefPtr<nsMediaList> mediaList(aMediaList);
+
+  if (!aMediaString.IsEmpty()) {
+    NS_ASSERTION(!aMediaList,
+                 "must not provide both aMediaString and aMediaList");
+    mediaList = new nsMediaList();
+
+    nsCSSParser mediumParser(this);
+
+    // We have aMediaString only when linked from link elements, style
+    // elements, or PIs, so pass true.
+    mediumParser.ParseMediaList(aMediaString, nullptr, 0, mediaList, true);
+  }
+
+  sheet->SetMedia(mediaList);
+
+  sheet->SetTitle(aTitle);
+  sheet->SetEnabled(!isAlternate);
+  sheet->SetScopeElement(aScopeElement);
+}
+
+/**
+ * InsertSheetInDoc handles ordering of sheets in the document.  Here
+ * we have two types of sheets -- those with linking elements and
+ * those without.  The latter are loaded by Link: headers.
+ * The following constraints are observed:
+ * 1) Any sheet with a linking element comes after all sheets without
+ *    linking elements
+ * 2) Sheets without linking elements are inserted in the order in
+ *    which the inserting requests come in, since all of these are
+ *    inserted during header data processing in the content sink
+ * 3) Sheets with linking elements are ordered based on document order
+ *    as determined by CompareDocumentPosition.
+ */
+nsresult
+Loader::InsertSheetInDoc(StyleSheet* aSheet,
+                         nsIContent* aLinkingContent,
+                         nsIDocument* aDocument)
+{
+  LOG(("css::Loader::InsertSheetInDoc"));
+  NS_PRECONDITION(aSheet, "Nothing to insert");
+  NS_PRECONDITION(aDocument, "Must have a document to insert into");
+
+  // XXX Need to cancel pending sheet loads for this element, if any
+
+  int32_t sheetCount = aDocument->GetNumberOfStyleSheets();
+
+  /*
+   * Start the walk at the _end_ of the list, since in the typical
+   * case we'll just want to append anyway.  We want to break out of
+   * the loop when insertionPoint points to just before the index we
+   * want to insert at.  In other words, when we leave the loop
+   * insertionPoint is the index of the stylesheet that immediately
+   * precedes the one we're inserting.
+   */
+  int32_t insertionPoint;
+  for (insertionPoint = sheetCount - 1; insertionPoint >= 0; --insertionPoint) {
+    StyleSheet* curSheet = aDocument->GetStyleSheetAt(insertionPoint);
+    NS_ASSERTION(curSheet, "There must be a sheet here!");
+    nsCOMPtr<nsINode> sheetOwner = curSheet->GetOwnerNode();
+    if (sheetOwner && !aLinkingContent) {
+      // Keep moving; all sheets with a sheetOwner come after all
+      // sheets without a linkingNode
+      continue;
+    }
+
+    if (!sheetOwner) {
+      // Aha!  The current sheet has no sheet owner, so we want to
+      // insert after it no matter whether we have a linkingNode
+      break;
+    }
+
+    NS_ASSERTION(aLinkingContent != sheetOwner,
+                 "Why do we still have our old sheet?");
+
+    // Have to compare
+    if (nsContentUtils::PositionIsBefore(sheetOwner, aLinkingContent)) {
+      // The current sheet comes before us, and it better be the first
+      // such, because now we break
+      break;
+    }
+  }
+
+  ++insertionPoint; // adjust the index to the spot we want to insert in
+
+  // XXX <meta> elements do not implement nsIStyleSheetLinkingElement;
+  // need to fix this for them to be ordered correctly.
+  nsCOMPtr<nsIStyleSheetLinkingElement>
+    linkingElement = do_QueryInterface(aLinkingContent);
+  if (linkingElement) {
+    linkingElement->SetStyleSheet(aSheet); // This sets the ownerNode on the sheet
+  }
+
+  aDocument->BeginUpdate(UPDATE_STYLE);
+  aDocument->InsertStyleSheetAt(aSheet, insertionPoint);
+  aDocument->EndUpdate(UPDATE_STYLE);
+  LOG(("  Inserting into document at position %d", insertionPoint));
+
+  return NS_OK;
+}
+
+/**
+ * InsertChildSheet handles ordering of @import-ed sheet in their
+ * parent sheets.  Here we want to just insert based on order of the
+ * @import rules that imported the sheets.  In theory we can't just
+ * append to the end because the CSSOM can insert @import rules.  In
+ * practice, we get the call to load the child sheet before the CSSOM
+ * has finished inserting the @import rule, so we have no idea where
+ * to put it anyway.  So just append for now.  (In the future if we
+ * want to insert the sheet at the correct position, we'll need to
+ * restore CSSStyleSheet::InsertStyleSheetAt, which was removed in
+ * bug 1220506.)
+ */
+nsresult
+Loader::InsertChildSheet(StyleSheet* aSheet,
+                         StyleSheet* aParentSheet,
+                         ImportRule* aParentRule)
+{
+  LOG(("css::Loader::InsertChildSheet"));
+  NS_PRECONDITION(aSheet, "Nothing to insert");
+  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->AsGecko()->SetEnabled(true);
+
+  aParentSheet->AppendStyleSheet(aSheet);
+  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));
+
+  return NS_OK;
+}
+
+/**
+ * LoadSheet handles the actual load of a sheet.  If the load is
+ * supposed to be synchronous it just opens a channel synchronously
+ * using the given uri, wraps the resulting stream in a converter
+ * stream and calls ParseSheet.  Otherwise it tries to look for an
+ * existing load for this URI and piggyback on it.  Failing all that,
+ * a new load is kicked off asynchronously.
+ */
+nsresult
+Loader::LoadSheet(SheetLoadData* aLoadData,
+                  StyleSheetState aSheetState,
+                  bool aIsPreload)
+{
+  LOG(("css::Loader::LoadSheet"));
+  NS_PRECONDITION(aLoadData, "Need a load data");
+  NS_PRECONDITION(aLoadData->mURI, "Need a URI to load");
+  NS_PRECONDITION(aLoadData->mSheet, "Need a sheet to load into");
+  NS_PRECONDITION(aSheetState != eSheetComplete, "Why bother?");
+  NS_PRECONDITION(!aLoadData->mUseSystemPrincipal || aLoadData->mSyncLoad,
+                  "Shouldn't use system principal for async loads");
+  NS_ASSERTION(mSheets, "mLoadingDatas should be initialized by now.");
+
+  LOG_URI("  Load from: '%s'", aLoadData->mURI);
+
+  nsresult rv = NS_OK;
+
+  if (!mDocument && !aLoadData->mIsNonDocumentSheet) {
+    // No point starting the load; just release all the data and such.
+    LOG_WARN(("  No document and not non-document sheet; pre-dropping load"));
+    SheetComplete(aLoadData, NS_BINDING_ABORTED);
+    return NS_BINDING_ABORTED;
+  }
+
+  SRIMetadata sriMetadata;
+  aLoadData->mSheet->GetIntegrity(sriMetadata);
+
+  if (aLoadData->mSyncLoad) {
+    LOG(("  Synchronous load"));
+    NS_ASSERTION(!aLoadData->mObserver, "Observer for a sync load?");
+    NS_ASSERTION(aSheetState == eSheetNeedsParser,
+                 "Sync loads can't reuse existing async loads");
+
+    // Create a nsIUnicharStreamLoader instance to which we will feed
+    // the data from the sync load.  Do this before creating the
+    // channel to make error recovery simpler.
+    nsCOMPtr<nsIUnicharStreamLoader> streamLoader;
+    rv = NS_NewUnicharStreamLoader(getter_AddRefs(streamLoader), aLoadData);
+    if (NS_FAILED(rv)) {
+      LOG_ERROR(("  Failed to create stream loader for sync load"));
+      SheetComplete(aLoadData, rv);
+      return rv;
+    }
+
+    if (mDocument) {
+      mozilla::net::PredictorLearn(aLoadData->mURI, mDocument->GetDocumentURI(),
+                                   nsINetworkPredictor::LEARN_LOAD_SUBRESOURCE,
+                                   mDocument);
+    }
+
+    nsSecurityFlags securityFlags =
+      nsILoadInfo::SEC_ALLOW_CROSS_ORIGIN_DATA_INHERITS |
+      nsILoadInfo::SEC_ALLOW_CHROME;
+
+    nsContentPolicyType contentPolicyType =
+      aIsPreload ? nsIContentPolicy::TYPE_INTERNAL_STYLESHEET_PRELOAD
+                 : nsIContentPolicy::TYPE_INTERNAL_STYLESHEET;
+
+    // Just load it
+    nsCOMPtr<nsIChannel> channel;
+    // Note that we are calling NS_NewChannelWithTriggeringPrincipal() with both
+    // a node and a principal.
+    // This is because of a case where the node is the document being styled and
+    // the principal is the stylesheet (perhaps from a different origin) that is
+    // applying the styles.
+    if (aLoadData->mRequestingNode && aLoadData->mLoaderPrincipal) {
+      rv = NS_NewChannelWithTriggeringPrincipal(getter_AddRefs(channel),
+                                                aLoadData->mURI,
+                                                aLoadData->mRequestingNode,
+                                                aLoadData->mLoaderPrincipal,
+                                                securityFlags,
+                                                contentPolicyType);
+    }
+    else {
+      // either we are loading something inside a document, in which case
+      // we should always have a requestingNode, or we are loading something
+      // outside a document, in which case the loadingPrincipal and the
+      // triggeringPrincipal should always be the systemPrincipal.
+      rv = NS_NewChannel(getter_AddRefs(channel),
+                         aLoadData->mURI,
+                         nsContentUtils::GetSystemPrincipal(),
+                         securityFlags,
+                         contentPolicyType);
+    }
+    if (NS_FAILED(rv)) {
+      LOG_ERROR(("  Failed to create channel"));
+      SheetComplete(aLoadData, rv);
+      return rv;
+    }
+
+    nsCOMPtr<nsIInputStream> stream;
+    rv = channel->Open2(getter_AddRefs(stream));
+
+    if (NS_FAILED(rv)) {
+      LOG_ERROR(("  Failed to open URI synchronously"));
+      SheetComplete(aLoadData, rv);
+      return rv;
+    }
+
+    // Force UA sheets to be UTF-8.
+    // XXX this is only necessary because the default in
+    // SheetLoadData::OnDetermineCharset is wrong (bug 521039).
+    channel->SetContentCharset(NS_LITERAL_CSTRING("UTF-8"));
+
+    // Manually feed the streamloader the contents of the stream.
+    // This will call back into OnStreamComplete
+    // and thence to ParseSheet.  Regardless of whether this fails,
+    // SheetComplete has been called.
+    return nsSyncLoadService::PushSyncStreamToListener(stream,
+                                                       streamLoader,
+                                                       channel);
+  }
+
+  SheetLoadData* existingData = nullptr;
+
+  URIPrincipalReferrerPolicyAndCORSModeHashKey key(aLoadData->mURI,
+                                     aLoadData->mLoaderPrincipal,
+                                     aLoadData->mSheet->GetCORSMode(),
+                                     aLoadData->mSheet->GetReferrerPolicy());
+  if (aSheetState == eSheetLoading) {
+    mSheets->mLoadingDatas.Get(&key, &existingData);
+    NS_ASSERTION(existingData, "CreateSheet lied about the state");
+  }
+  else if (aSheetState == eSheetPending){
+    mSheets->mPendingDatas.Get(&key, &existingData);
+    NS_ASSERTION(existingData, "CreateSheet lied about the state");
+  }
+
+  if (existingData) {
+    LOG(("  Glomming on to existing load"));
+    SheetLoadData* data = existingData;
+    while (data->mNext) {
+      data = data->mNext;
+    }
+    data->mNext = aLoadData; // transfer ownership
+    if (aSheetState == eSheetPending && !aLoadData->mWasAlternate) {
+      // Kick the load off; someone cares about it right away
+
+#ifdef DEBUG
+      SheetLoadData* removedData;
+      NS_ASSERTION(mSheets->mPendingDatas.Get(&key, &removedData) &&
+                   removedData == existingData,
+                   "Bad pending table.");
+#endif
+
+      mSheets->mPendingDatas.Remove(&key);
+
+      LOG(("  Forcing load of pending data"));
+      return LoadSheet(existingData, eSheetNeedsParser, aIsPreload);
+    }
+    // All done here; once the load completes we'll be marked complete
+    // automatically
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsILoadGroup> loadGroup;
+  if (mDocument) {
+    loadGroup = mDocument->GetDocumentLoadGroup();
+    // load for a document with no loadgrup indicates that something is
+    // completely bogus, let's bail out early.
+    if (!loadGroup) {
+      LOG_ERROR(("  Failed to query loadGroup from document"));
+      SheetComplete(aLoadData, NS_ERROR_UNEXPECTED);
+      return NS_ERROR_UNEXPECTED;
+    }
+  }
+#ifdef DEBUG
+  mSyncCallback = true;
+#endif
+
+  CORSMode ourCORSMode = aLoadData->mSheet->GetCORSMode();
+  nsSecurityFlags securityFlags =
+    ourCORSMode == CORS_NONE
+      ? nsILoadInfo::SEC_ALLOW_CROSS_ORIGIN_DATA_INHERITS
+      : nsILoadInfo::SEC_REQUIRE_CORS_DATA_INHERITS;
+  if (ourCORSMode == CORS_ANONYMOUS) {
+    securityFlags |= nsILoadInfo::SEC_COOKIES_SAME_ORIGIN;
+  } else if (ourCORSMode == CORS_USE_CREDENTIALS) {
+    securityFlags |= nsILoadInfo::SEC_COOKIES_INCLUDE;
+  }
+  securityFlags |= nsILoadInfo::SEC_ALLOW_CHROME;
+
+  nsContentPolicyType contentPolicyType =
+    aIsPreload ? nsIContentPolicy::TYPE_INTERNAL_STYLESHEET_PRELOAD
+               : nsIContentPolicy::TYPE_INTERNAL_STYLESHEET;
+
+  nsCOMPtr<nsIChannel> channel;
+  // Note we are calling NS_NewChannelWithTriggeringPrincipal here with a node
+  // and a principal. This is because of a case where the node is the document
+  // being styled and the principal is the stylesheet (perhaps from a different
+  // origin)  that is applying the styles.
+  if (aLoadData->mRequestingNode && aLoadData->mLoaderPrincipal) {
+    rv = NS_NewChannelWithTriggeringPrincipal(getter_AddRefs(channel),
+                                              aLoadData->mURI,
+                                              aLoadData->mRequestingNode,
+                                              aLoadData->mLoaderPrincipal,
+                                              securityFlags,
+                                              contentPolicyType,
+                                              loadGroup,
+                                              nullptr,   // aCallbacks
+                                              nsIChannel::LOAD_NORMAL |
+                                              nsIChannel::LOAD_CLASSIFY_URI);
+  }
+  else {
+    // either we are loading something inside a document, in which case
+    // we should always have a requestingNode, or we are loading something
+    // outside a document, in which case the loadingPrincipal and the
+    // triggeringPrincipal should always be the systemPrincipal.
+    rv = NS_NewChannel(getter_AddRefs(channel),
+                       aLoadData->mURI,
+                       nsContentUtils::GetSystemPrincipal(),
+                       securityFlags,
+                       contentPolicyType,
+                       loadGroup,
+                       nullptr,   // aCallbacks
+                       nsIChannel::LOAD_NORMAL |
+                       nsIChannel::LOAD_CLASSIFY_URI);
+  }
+
+  if (NS_FAILED(rv)) {
+#ifdef DEBUG
+    mSyncCallback = false;
+#endif
+    LOG_ERROR(("  Failed to create channel"));
+    SheetComplete(aLoadData, rv);
+    return rv;
+  }
+
+  if (!aLoadData->mWasAlternate) {
+    nsCOMPtr<nsIClassOfService> cos(do_QueryInterface(channel));
+    if (cos) {
+      cos->AddClassFlags(nsIClassOfService::Leader);
+    }
+  }
+
+  nsCOMPtr<nsIHttpChannel> httpChannel(do_QueryInterface(channel));
+  if (httpChannel) {
+    // Send a minimal Accept header for text/css
+    httpChannel->SetRequestHeader(NS_LITERAL_CSTRING("Accept"),
+                                  NS_LITERAL_CSTRING("text/css,*/*;q=0.1"),
+                                  false);
+    nsCOMPtr<nsIURI> referrerURI = aLoadData->GetReferrerURI();
+    if (referrerURI)
+      httpChannel->SetReferrerWithPolicy(referrerURI,
+                                         aLoadData->mSheet->GetReferrerPolicy());
+
+    nsCOMPtr<nsIHttpChannelInternal> internalChannel = do_QueryInterface(httpChannel);
+    if (internalChannel) {
+      internalChannel->SetIntegrityMetadata(sriMetadata.GetIntegrityString());
+    }
+
+    // Set the initiator type
+    nsCOMPtr<nsITimedChannel> timedChannel(do_QueryInterface(httpChannel));
+    if (timedChannel) {
+      if (aLoadData->mParentData) {
+        timedChannel->SetInitiatorType(NS_LITERAL_STRING("css"));
+      } else {
+        timedChannel->SetInitiatorType(NS_LITERAL_STRING("link"));
+      }
+    }
+  }
+
+  // Now tell the channel we expect text/css data back....  We do
+  // this before opening it, so it's only treated as a hint.
+  channel->SetContentType(NS_LITERAL_CSTRING("text/css"));
+
+  // We don't have to hold on to the stream loader.  The ownership
+  // model is: Necko owns the stream loader, which owns the load data,
+  // which owns us
+  nsCOMPtr<nsIUnicharStreamLoader> streamLoader;
+  rv = NS_NewUnicharStreamLoader(getter_AddRefs(streamLoader), aLoadData);
+  if (NS_FAILED(rv)) {
+#ifdef DEBUG
+    mSyncCallback = false;
+#endif
+    LOG_ERROR(("  Failed to create stream loader"));
+    SheetComplete(aLoadData, rv);
+    return rv;
+  }
+
+  if (mDocument) {
+    mozilla::net::PredictorLearn(aLoadData->mURI, mDocument->GetDocumentURI(),
+                                 nsINetworkPredictor::LEARN_LOAD_SUBRESOURCE,
+                                 mDocument);
+  }
+
+  rv = channel->AsyncOpen2(streamLoader);
+
+#ifdef DEBUG
+  mSyncCallback = false;
+#endif
+
+  if (NS_FAILED(rv)) {
+    LOG_ERROR(("  Failed to create stream loader"));
+    SheetComplete(aLoadData, rv);
+    return rv;
+  }
+
+  mSheets->mLoadingDatas.Put(&key, aLoadData);
+  aLoadData->mIsLoading = true;
+
+  return NS_OK;
+}
+
+/**
+ * ParseSheet handles parsing the data stream.  The main idea here is
+ * to push the current load data onto the parse stack before letting
+ * the CSS parser at the data stream.  That lets us handle @import
+ * correctly.
+ */
+nsresult
+Loader::ParseSheet(const nsAString& aInput,
+                   SheetLoadData* aLoadData,
+                   bool& aCompleted)
+{
+  LOG(("css::Loader::ParseSheet"));
+  NS_PRECONDITION(aLoadData, "Must have load data");
+  NS_PRECONDITION(aLoadData->mSheet, "Must have sheet to parse into");
+
+  aCompleted = false;
+
+  // Push our load data on the stack so any kids can pick it up
+  mParsingDatas.AppendElement(aLoadData);
+  nsIURI* sheetURI = aLoadData->mSheet->GetSheetURI();
+  nsIURI* baseURI = aLoadData->mSheet->GetBaseURI();
+
+  nsresult rv;
+
+  if (aLoadData->mSheet->IsGecko()) {
+    nsCSSParser parser(this, aLoadData->mSheet->AsGecko());
+    rv = parser.ParseSheet(aInput, sheetURI, baseURI,
+                           aLoadData->mSheet->Principal(),
+                           aLoadData->mLineNumber);
+  } else {
+    rv =
+      aLoadData->mSheet->AsServo()->ParseSheet(aInput, sheetURI, baseURI,
+                                               aLoadData->mSheet->Principal(),
+                                               aLoadData->mLineNumber);
+  }
+
+  mParsingDatas.RemoveElementAt(mParsingDatas.Length() - 1);
+
+  if (NS_FAILED(rv)) {
+    LOG_ERROR(("  Low-level error in parser!"));
+    SheetComplete(aLoadData, rv);
+    return rv;
+  }
+
+  NS_ASSERTION(aLoadData->mPendingChildren == 0 || !aLoadData->mSyncLoad,
+               "Sync load has leftover pending children!");
+
+  if (aLoadData->mPendingChildren == 0) {
+    LOG(("  No pending kids from parse"));
+    aCompleted = true;
+    SheetComplete(aLoadData, NS_OK);
+  }
+  // Otherwise, the children are holding strong refs to the data and
+  // will call SheetComplete() on it when they complete.
+
+  return NS_OK;
+}
+
+/**
+ * SheetComplete is the do-it-all cleanup function.  It removes the
+ * load data from the "loading" hashtable, adds the sheet to the
+ * "completed" hashtable, massages the XUL cache, handles siblings of
+ * the load data (other loads for the same URI), handles unblocking
+ * blocked parent loads as needed, and most importantly calls
+ * NS_RELEASE on the load data to destroy the whole mess.
+ */
+void
+Loader::SheetComplete(SheetLoadData* aLoadData, nsresult aStatus)
+{
+  LOG(("css::Loader::SheetComplete"));
+
+  if (aLoadData->mSheet->IsServo() && NS_FAILED(aStatus)) {
+    aLoadData->mSheet->AsServo()->LoadFailed();
+  }
+
+  // 8 is probably big enough for all our common cases.  It's not likely that
+  // imports will nest more than 8 deep, and multiple sheets with the same URI
+  // are rare.
+  AutoTArray<RefPtr<SheetLoadData>, 8> datasToNotify;
+  DoSheetComplete(aLoadData, aStatus, datasToNotify);
+
+  // Now it's safe to go ahead and notify observers
+  uint32_t count = datasToNotify.Length();
+  mDatasToNotifyOn += count;
+  for (uint32_t i = 0; i < count; ++i) {
+    --mDatasToNotifyOn;
+
+    SheetLoadData* data = datasToNotify[i];
+    NS_ASSERTION(data && data->mMustNotify, "How did this data get here?");
+    if (data->mObserver) {
+      LOG(("  Notifying observer %p for data %p.  wasAlternate: %d",
+           data->mObserver.get(), data, data->mWasAlternate));
+      data->mObserver->StyleSheetLoaded(data->mSheet, data->mWasAlternate,
+                                        aStatus);
+    }
+
+    nsTObserverArray<nsCOMPtr<nsICSSLoaderObserver> >::ForwardIterator iter(mObservers);
+    nsCOMPtr<nsICSSLoaderObserver> obs;
+    while (iter.HasMore()) {
+      obs = iter.GetNext();
+      LOG(("  Notifying global observer %p for data %p.  wasAlternate: %d",
+           obs.get(), data, data->mWasAlternate));
+      obs->StyleSheetLoaded(data->mSheet, data->mWasAlternate, aStatus);
+    }
+  }
+
+  if (mSheets->mLoadingDatas.Count() == 0 && mSheets->mPendingDatas.Count() > 0) {
+    LOG(("  No more loading sheets; starting alternates"));
+    StartAlternateLoads();
+  }
+}
+
+void
+Loader::DoSheetComplete(SheetLoadData* aLoadData, nsresult aStatus,
+                        LoadDataArray& aDatasToNotify)
+{
+  LOG(("css::Loader::DoSheetComplete"));
+  NS_PRECONDITION(aLoadData, "Must have a load data!");
+  NS_PRECONDITION(aLoadData->mSheet, "Must have a sheet");
+  NS_ASSERTION(mSheets, "mLoadingDatas should be initialized by now.");
+
+  LOG(("Load completed, status: 0x%x", aStatus));
+
+  // Twiddle the hashtables
+  if (aLoadData->mURI) {
+    LOG_URI("  Finished loading: '%s'", aLoadData->mURI);
+    // Remove the data from the list of loading datas
+    if (aLoadData->mIsLoading) {
+      URIPrincipalReferrerPolicyAndCORSModeHashKey key(aLoadData->mURI,
+                                         aLoadData->mLoaderPrincipal,
+                                         aLoadData->mSheet->GetCORSMode(),
+                                         aLoadData->mSheet->GetReferrerPolicy());
+#ifdef DEBUG
+      SheetLoadData *loadingData;
+      NS_ASSERTION(mSheets->mLoadingDatas.Get(&key, &loadingData) &&
+                   loadingData == aLoadData,
+                   "Bad loading table");
+#endif
+
+      mSheets->mLoadingDatas.Remove(&key);
+      aLoadData->mIsLoading = false;
+    }
+  }
+
+  // Go through and deal with the whole linked list.
+  SheetLoadData* data = aLoadData;
+  while (data) {
+    if (!data->mSheetAlreadyComplete) {
+      // 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->IsGecko() &&
+                   data->mSheet->AsGecko()->IsModified()),
+                 "should not get marked modified during parsing");
+      data->mSheet->SetComplete();
+      data->ScheduleLoadEventIfNeeded(aStatus);
+    }
+    if (data->mMustNotify && (data->mObserver || !mObservers.IsEmpty())) {
+      // Don't notify here so we don't trigger script.  Remember the
+      // info we need to notify, then do it later when it's safe.
+      aDatasToNotify.AppendElement(data);
+
+      // On append failure, just press on.  We'll fail to notify the observer,
+      // but not much we can do about that....
+    }
+
+    NS_ASSERTION(!data->mParentData ||
+                 data->mParentData->mPendingChildren != 0,
+                 "Broken pending child count on our parent");
+
+    // If we have a parent, our parent is no longer being parsed, and
+    // we are the last pending child, then our load completion
+    // completes the parent too.  Note that the parent _can_ still be
+    // being parsed (eg if the child (us) failed to open the channel
+    // or some such).
+    if (data->mParentData &&
+        --(data->mParentData->mPendingChildren) == 0 &&
+        !mParsingDatas.Contains(data->mParentData)) {
+      DoSheetComplete(data->mParentData, aStatus, aDatasToNotify);
+    }
+
+    data = data->mNext;
+  }
+
+  // Now that it's marked complete, put the sheet in our cache.
+  // If we ever start doing this for failure aStatus, we'll need to
+  // adjust the PostLoadEvent code that thinks anything already
+  // complete must have loaded succesfully.
+  if (NS_SUCCEEDED(aStatus) && aLoadData->mURI) {
+    // Pick our sheet to cache carefully.  Ideally, we want to cache
+    // 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.
+    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;
+      }
+#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);
+          }
+        }
+      }
+      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);
+#ifdef MOZ_XUL
+      }
+#endif
+    } else {
+      NS_WARNING("stylo: Stylesheet caching not yet supported - see bug 1290218.");
+    }
+  }
+
+  NS_RELEASE(aLoadData);  // this will release parents and siblings and all that
+}
+
+nsresult
+Loader::LoadInlineStyle(nsIContent* aElement,
+                        const nsAString& aBuffer,
+                        uint32_t aLineNumber,
+                        const nsAString& aTitle,
+                        const nsAString& aMedia,
+                        Element* aScopeElement,
+                        nsICSSLoaderObserver* aObserver,
+                        bool* aCompleted,
+                        bool* aIsAlternate)
+{
+  LOG(("css::Loader::LoadInlineStyle"));
+  NS_ASSERTION(mParsingDatas.Length() == 0, "We're in the middle of a parse?");
+
+  *aCompleted = true;
+
+  if (!mEnabled) {
+    LOG_WARN(("  Not enabled"));
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  NS_ENSURE_TRUE(mDocument, NS_ERROR_NOT_INITIALIZED);
+
+  nsCOMPtr<nsIStyleSheetLinkingElement> owningElement(do_QueryInterface(aElement));
+  NS_ASSERTION(owningElement, "Element is not a style linking element!");
+
+  // Since we're not planning to load a URI, no need to hand a principal to the
+  // load data or to CreateSheet().  Also, OK to use CORS_NONE for the CORS
+  // mode and mDocument's ReferrerPolicy.
+  StyleSheetState state;
+  RefPtr<StyleSheet> sheet;
+  nsresult rv = CreateSheet(nullptr, aElement, nullptr, eAuthorSheetFeatures,
+                            CORS_NONE, mDocument->GetReferrerPolicy(),
+                            EmptyString(), // no inline integrity checks
+                            false, false, aTitle, state, aIsAlternate,
+                            &sheet);
+  NS_ENSURE_SUCCESS(rv, rv);
+  NS_ASSERTION(state == eSheetNeedsParser,
+               "Inline sheets should not be cached");
+
+  LOG(("  Sheet is alternate: %d", *aIsAlternate));
+
+  PrepareSheet(sheet, aTitle, aMedia, nullptr, aScopeElement, *aIsAlternate);
+
+  if (aElement->HasFlag(NODE_IS_IN_SHADOW_TREE)) {
+    ShadowRoot* containingShadow = aElement->GetContainingShadow();
+    MOZ_ASSERT(containingShadow);
+    containingShadow->InsertSheet(sheet, aElement);
+  } else {
+    rv = InsertSheetInDoc(sheet, aElement, mDocument);
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+
+  SheetLoadData* data = new SheetLoadData(this, aTitle, nullptr, sheet,
+                                          owningElement, *aIsAlternate,
+                                          aObserver, nullptr, static_cast<nsINode*>(aElement));
+
+  // We never actually load this, so just set its principal directly
+  sheet->SetPrincipal(aElement->NodePrincipal());
+
+  NS_ADDREF(data);
+  data->mLineNumber = aLineNumber;
+  // Parse completion releases the load data
+  rv = ParseSheet(aBuffer, data, *aCompleted);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // If aCompleted is true, |data| may well be deleted by now.
+  if (!*aCompleted) {
+    data->mMustNotify = true;
+  }
+  return rv;
+}
+
+nsresult
+Loader::LoadStyleLink(nsIContent* aElement,
+                      nsIURI* aURL,
+                      const nsAString& aTitle,
+                      const nsAString& aMedia,
+                      bool aHasAlternateRel,
+                      CORSMode aCORSMode,
+                      ReferrerPolicy aReferrerPolicy,
+                      const nsAString& aIntegrity,
+                      nsICSSLoaderObserver* aObserver,
+                      bool* aIsAlternate)
+{
+  LOG(("css::Loader::LoadStyleLink"));
+  NS_PRECONDITION(aURL, "Must have URL to load");
+  NS_ASSERTION(mParsingDatas.Length() == 0, "We're in the middle of a parse?");
+
+  LOG_URI("  Link uri: '%s'", aURL);
+  LOG(("  Link title: '%s'", NS_ConvertUTF16toUTF8(aTitle).get()));
+  LOG(("  Link media: '%s'", NS_ConvertUTF16toUTF8(aMedia).get()));
+  LOG(("  Link alternate rel: %d", aHasAlternateRel));
+
+  if (!mEnabled) {
+    LOG_WARN(("  Not enabled"));
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  NS_ENSURE_TRUE(mDocument, NS_ERROR_NOT_INITIALIZED);
+
+  nsIPrincipal* principal =
+    aElement ? aElement->NodePrincipal() : mDocument->NodePrincipal();
+
+  nsISupports* context = aElement;
+  if (!context) {
+    context = mDocument;
+  }
+
+  nsresult rv = CheckContentPolicy(principal, aURL, context, false);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    // Don't fire the error event if our document is loaded as data.  We're
+    // supposed to not even try to do loads in that case... Unfortunately, we
+    // implement that via nsDataDocumentContentPolicy, which doesn't have a good
+    // way to communicate back to us that _it_ is the thing that blocked the
+    // load.
+    if (aElement && !mDocument->IsLoadedAsData()) {
+      // Fire an async error event on it.
+      RefPtr<AsyncEventDispatcher> loadBlockingAsyncDispatcher =
+        new LoadBlockingAsyncEventDispatcher(aElement,
+                                             NS_LITERAL_STRING("error"),
+                                             false, false);
+      loadBlockingAsyncDispatcher->PostDOMEvent();
+    }
+    return rv;
+  }
+
+  StyleSheetState state;
+  RefPtr<StyleSheet> sheet;
+  rv = CreateSheet(aURL, aElement, principal, eAuthorSheetFeatures,
+                   aCORSMode, aReferrerPolicy, aIntegrity, false,
+                   aHasAlternateRel, aTitle, state, aIsAlternate,
+                   &sheet);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  LOG(("  Sheet is alternate: %d", *aIsAlternate));
+
+  PrepareSheet(sheet, aTitle, aMedia, nullptr, nullptr, *aIsAlternate);
+
+  rv = InsertSheetInDoc(sheet, aElement, mDocument);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsCOMPtr<nsIStyleSheetLinkingElement> owningElement(do_QueryInterface(aElement));
+
+  if (state == eSheetComplete) {
+    LOG(("  Sheet already complete: 0x%p", sheet.get()));
+    if (aObserver || !mObservers.IsEmpty() || owningElement) {
+      rv = PostLoadEvent(aURL, sheet, aObserver, *aIsAlternate,
+                         owningElement);
+      return rv;
+    }
+
+    return NS_OK;
+  }
+
+  // Now we need to actually load it
+  nsCOMPtr<nsINode> requestingNode = do_QueryInterface(context);
+  SheetLoadData* data = new SheetLoadData(this, aTitle, aURL, sheet,
+                                          owningElement, *aIsAlternate,
+                                          aObserver, principal, requestingNode);
+  NS_ADDREF(data);
+
+  // If we have to parse and it's an alternate non-inline, defer it
+  if (aURL && state == eSheetNeedsParser && mSheets->mLoadingDatas.Count() != 0 &&
+      *aIsAlternate) {
+    LOG(("  Deferring alternate sheet load"));
+    URIPrincipalReferrerPolicyAndCORSModeHashKey key(data->mURI,
+                                                     data->mLoaderPrincipal,
+                                                     data->mSheet->GetCORSMode(),
+                                                     data->mSheet->GetReferrerPolicy());
+    mSheets->mPendingDatas.Put(&key, data);
+
+    data->mMustNotify = true;
+    return NS_OK;
+  }
+
+  // Load completion will free the data
+  rv = LoadSheet(data, state, false);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  data->mMustNotify = true;
+  return rv;
+}
+
+static bool
+HaveAncestorDataWithURI(SheetLoadData *aData, nsIURI *aURI)
+{
+  if (!aData->mURI) {
+    // Inline style; this won't have any ancestors
+    MOZ_ASSERT(!aData->mParentData,
+               "How does inline style have a parent?");
+    return false;
+  }
+
+  bool equal;
+  if (NS_FAILED(aData->mURI->Equals(aURI, &equal)) || equal) {
+    return true;
+  }
+
+  // Datas down the mNext chain have the same URI as aData, so we
+  // don't have to compare to them.  But they might have different
+  // parents, and we have to check all of those.
+  while (aData) {
+    if (aData->mParentData &&
+        HaveAncestorDataWithURI(aData->mParentData, aURI)) {
+      return true;
+    }
+
+    aData = aData->mNext;
+  }
+
+  return false;
+}
+
+nsresult
+Loader::LoadChildSheet(StyleSheet* aParentSheet,
+                       nsIURI* aURL,
+                       nsMediaList* aMedia,
+                       ImportRule* aParentRule,
+                       LoaderReusableStyleSheets* aReusableSheets)
+{
+  LOG(("css::Loader::LoadChildSheet"));
+  NS_PRECONDITION(aURL, "Must have a URI to load");
+  NS_PRECONDITION(aParentSheet, "Must have a parent sheet");
+
+  if (!mEnabled) {
+    LOG_WARN(("  Not enabled"));
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  LOG_URI("  Child uri: '%s'", aURL);
+
+  nsCOMPtr<nsINode> owningNode;
+
+  // check for an owning document: if none, don't bother walking up the parent
+  // sheets
+  if (aParentSheet->GetOwningDocument()) {
+    StyleSheet* topSheet = aParentSheet;
+    while (StyleSheet* parent = topSheet->GetParentSheet()) {
+      topSheet = parent;
+    }
+    owningNode = topSheet->GetOwnerNode();
+  }
+
+  nsISupports* context = owningNode;
+  if (!context) {
+    context = mDocument;
+  }
+
+  nsIPrincipal* principal = aParentSheet->Principal();
+  nsresult rv = CheckContentPolicy(principal, aURL, context, false);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  SheetLoadData* parentData = nullptr;
+  nsCOMPtr<nsICSSLoaderObserver> observer;
+
+  int32_t count = mParsingDatas.Length();
+  if (count > 0) {
+    LOG(("  Have a parent load"));
+    parentData = mParsingDatas.ElementAt(count - 1);
+    // Check for cycles
+    if (HaveAncestorDataWithURI(parentData, aURL)) {
+      // Houston, we have a loop, blow off this child and pretend this never
+      // happened
+      LOG_ERROR(("  @import cycle detected, dropping load"));
+      return NS_OK;
+    }
+
+    NS_ASSERTION(parentData->mSheet == aParentSheet,
+                 "Unexpected call to LoadChildSheet");
+  } else {
+    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.
+    // 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<StyleSheet> sheet;
+  RefPtr<CSSStyleSheet> reusableSheet;
+  StyleSheetState state;
+  if (aReusableSheets && aReusableSheets->FindReusableStyleSheet(aURL, reusableSheet)) {
+    sheet = reusableSheet;
+    aParentRule->SetSheet(reusableSheet);
+    state = eSheetComplete;
+  } else {
+    bool isAlternate;
+    const nsSubstring& empty = EmptyString();
+    // For now, use CORS_NONE for child sheets
+    rv = CreateSheet(aURL, nullptr, principal,
+                     aParentSheet->ParsingMode(),
+                     CORS_NONE, aParentSheet->GetReferrerPolicy(),
+                     EmptyString(), // integrity is only checked on main sheet
+                     parentData ? parentData->mSyncLoad : false,
+                     false, empty, state, &isAlternate, &sheet);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    PrepareSheet(sheet, empty, empty, aMedia, nullptr, isAlternate);
+  }
+
+  rv = InsertChildSheet(sheet, aParentSheet, aParentRule);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (state == eSheetComplete) {
+    LOG(("  Sheet already complete"));
+    // We're completely done.  No need to notify, even, since the
+    // @import rule addition/modification will trigger the right style
+    // changes automatically.
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsINode> requestingNode = do_QueryInterface(context);
+  SheetLoadData* data = new SheetLoadData(this, aURL, sheet, parentData,
+                                          observer, principal, requestingNode);
+
+  NS_ADDREF(data);
+  bool syncLoad = data->mSyncLoad;
+
+  // Load completion will release the data
+  rv = LoadSheet(data, state, false);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // If syncLoad is true, |data| will be deleted by now.
+  if (!syncLoad) {
+    data->mMustNotify = true;
+  }
+  return rv;
+}
+
+nsresult
+Loader::LoadSheetSync(nsIURI* aURL,
+                      SheetParsingMode aParsingMode,
+                      bool aUseSystemPrincipal,
+                      RefPtr<StyleSheet>* aSheet)
+{
+  LOG(("css::Loader::LoadSheetSync"));
+  return InternalLoadNonDocumentSheet(aURL,
+                                      false, aParsingMode, aUseSystemPrincipal,
+                                      nullptr, EmptyCString(),
+                                      aSheet, nullptr);
+}
+
+nsresult
+Loader::LoadSheet(nsIURI* aURL,
+                  nsIPrincipal* aOriginPrincipal,
+                  const nsCString& aCharset,
+                  nsICSSLoaderObserver* aObserver,
+                  RefPtr<StyleSheet>* aSheet)
+{
+  LOG(("css::Loader::LoadSheet(aURL, aObserver, aSheet) api call"));
+  NS_PRECONDITION(aSheet, "aSheet is null");
+  return InternalLoadNonDocumentSheet(aURL,
+                                      false, eAuthorSheetFeatures, false,
+                                      aOriginPrincipal, aCharset,
+                                      aSheet, aObserver);
+}
+
+nsresult
+Loader::LoadSheet(nsIURI* aURL,
+                  bool aIsPreload,
+                  nsIPrincipal* aOriginPrincipal,
+                  const nsCString& aCharset,
+                  nsICSSLoaderObserver* aObserver,
+                  CORSMode aCORSMode,
+                  ReferrerPolicy aReferrerPolicy,
+                  const nsAString& aIntegrity)
+{
+  LOG(("css::Loader::LoadSheet(aURL, aObserver) api call"));
+  return InternalLoadNonDocumentSheet(aURL,
+                                      aIsPreload, eAuthorSheetFeatures, false,
+                                      aOriginPrincipal, aCharset,
+                                      nullptr, aObserver,
+                                      aCORSMode, aReferrerPolicy, aIntegrity);
+}
+
+nsresult
+Loader::InternalLoadNonDocumentSheet(nsIURI* aURL,
+                                     bool aIsPreload,
+                                     SheetParsingMode aParsingMode,
+                                     bool aUseSystemPrincipal,
+                                     nsIPrincipal* aOriginPrincipal,
+                                     const nsCString& aCharset,
+                                     RefPtr<StyleSheet>* aSheet,
+                                     nsICSSLoaderObserver* aObserver,
+                                     CORSMode aCORSMode,
+                                     ReferrerPolicy aReferrerPolicy,
+                                     const nsAString& aIntegrity)
+{
+  NS_PRECONDITION(aURL, "Must have a URI to load");
+  NS_PRECONDITION(aSheet || aObserver, "Sheet and observer can't both be null");
+  NS_PRECONDITION(!aUseSystemPrincipal || !aObserver,
+                  "Shouldn't load system-principal sheets async");
+  NS_ASSERTION(mParsingDatas.Length() == 0, "We're in the middle of a parse?");
+
+  LOG_URI("  Non-document sheet uri: '%s'", aURL);
+
+  if (aSheet) {
+    *aSheet = nullptr;
+  }
+
+  if (!mEnabled) {
+    LOG_WARN(("  Not enabled"));
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  nsresult rv = CheckContentPolicy(aOriginPrincipal, aURL, mDocument, aIsPreload);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  StyleSheetState state;
+  bool isAlternate;
+  RefPtr<StyleSheet> sheet;
+  bool syncLoad = (aObserver == nullptr);
+  const nsSubstring& empty = EmptyString();
+
+  rv = CreateSheet(aURL, nullptr, aOriginPrincipal, aParsingMode,
+                   aCORSMode, aReferrerPolicy, aIntegrity, syncLoad,
+                   false, empty, state, &isAlternate, &sheet);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  PrepareSheet(sheet, empty, empty, nullptr, nullptr, isAlternate);
+
+  if (state == eSheetComplete) {
+    LOG(("  Sheet already complete"));
+    if (aObserver || !mObservers.IsEmpty()) {
+      rv = PostLoadEvent(aURL, sheet, aObserver, false, nullptr);
+    }
+    if (aSheet) {
+      sheet.swap(*aSheet);
+    }
+    return rv;
+  }
+
+  SheetLoadData* data =
+    new SheetLoadData(this, aURL, sheet, syncLoad,
+                      aUseSystemPrincipal, aCharset, aObserver,
+                      aOriginPrincipal, mDocument);
+
+  NS_ADDREF(data);
+  rv = LoadSheet(data, state, aIsPreload);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (aSheet) {
+    sheet.swap(*aSheet);
+  }
+  if (aObserver) {
+    data->mMustNotify = true;
+  }
+
+  return rv;
+}
+
+nsresult
+Loader::PostLoadEvent(nsIURI* aURI,
+                      StyleSheet* aSheet,
+                      nsICSSLoaderObserver* aObserver,
+                      bool aWasAlternate,
+                      nsIStyleSheetLinkingElement* aElement)
+{
+  LOG(("css::Loader::PostLoadEvent"));
+  NS_PRECONDITION(aSheet, "Must have sheet");
+  NS_PRECONDITION(aObserver || !mObservers.IsEmpty() || aElement,
+                  "Must have observer or element");
+
+  RefPtr<SheetLoadData> evt =
+    new SheetLoadData(this, EmptyString(), // title doesn't matter here
+                      aURI,
+                      aSheet,
+                      aElement,
+                      aWasAlternate,
+                      aObserver,
+                      nullptr,
+                      mDocument);
+  NS_ENSURE_TRUE(evt, NS_ERROR_OUT_OF_MEMORY);
+
+  if (!mPostedEvents.AppendElement(evt)) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  nsresult rv = NS_DispatchToCurrentThread(evt);
+  if (NS_FAILED(rv)) {
+    NS_WARNING("failed to dispatch stylesheet load event");
+    mPostedEvents.RemoveElement(evt);
+  } else {
+    // We'll unblock onload when we handle the event.
+    if (mDocument) {
+      mDocument->BlockOnload();
+    }
+
+    // We want to notify the observer for this data.
+    evt->mMustNotify = true;
+    evt->mSheetAlreadyComplete = true;
+
+    // If we get to this code, aSheet loaded correctly at some point, so
+    // we can just use NS_OK for the status.  Note that we do this here
+    // and not from inside our SheetComplete so that we don't end up
+    // running the load event async.
+    evt->ScheduleLoadEventIfNeeded(NS_OK);
+  }
+
+  return rv;
+}
+
+void
+Loader::HandleLoadEvent(SheetLoadData* aEvent)
+{
+  // XXXbz can't assert this yet.... May not have an observer because
+  // we're unblocking the parser
+  // NS_ASSERTION(aEvent->mObserver, "Must have observer");
+  NS_ASSERTION(aEvent->mSheet, "Must have sheet");
+
+  // Very important: this needs to come before the SheetComplete call
+  // below, so that HasPendingLoads() will test true as needed under
+  // notifications we send from that SheetComplete call.
+  mPostedEvents.RemoveElement(aEvent);
+
+  if (!aEvent->mIsCancelled) {
+    // SheetComplete will call Release(), so give it a reference to do
+    // that with.
+    NS_ADDREF(aEvent);
+    SheetComplete(aEvent, NS_OK);
+  }
+
+  if (mDocument) {
+    mDocument->UnblockOnload(true);
+  }
+}
+
+static void
+StopLoadingSheets(
+  nsDataHashtable<URIPrincipalReferrerPolicyAndCORSModeHashKey, SheetLoadData*>& aDatas,
+  Loader::LoadDataArray& aArr)
+{
+  for (auto iter = aDatas.Iter(); !iter.Done(); iter.Next()) {
+    SheetLoadData* data = iter.Data();
+    MOZ_ASSERT(data, "Must have a data!");
+
+    data->mIsLoading = false; // we will handle the removal right here
+    data->mIsCancelled = true;
+
+    aArr.AppendElement(data);
+
+    iter.Remove();
+  }
+}
+
+nsresult
+Loader::Stop()
+{
+  uint32_t pendingCount = mSheets ? mSheets->mPendingDatas.Count() : 0;
+  uint32_t loadingCount = mSheets ? mSheets->mLoadingDatas.Count() : 0;
+  LoadDataArray arr(pendingCount + loadingCount + mPostedEvents.Length());
+
+  if (pendingCount) {
+    StopLoadingSheets(mSheets->mPendingDatas, arr);
+  }
+  if (loadingCount) {
+    StopLoadingSheets(mSheets->mLoadingDatas, arr);
+  }
+
+  uint32_t i;
+  for (i = 0; i < mPostedEvents.Length(); ++i) {
+    SheetLoadData* data = mPostedEvents[i];
+    data->mIsCancelled = true;
+    if (arr.AppendElement(data)) {
+      // SheetComplete() calls Release(), so give this an extra ref.
+      NS_ADDREF(data);
+    }
+#ifdef DEBUG
+    else {
+      NS_NOTREACHED("We preallocated this memory... shouldn't really fail, "
+                    "except we never check that preallocation succeeds.");
+    }
+#endif
+  }
+  mPostedEvents.Clear();
+
+  mDatasToNotifyOn += arr.Length();
+  for (i = 0; i < arr.Length(); ++i) {
+    --mDatasToNotifyOn;
+    SheetComplete(arr[i], NS_BINDING_ABORTED);
+  }
+  return NS_OK;
+}
+
+bool
+Loader::HasPendingLoads()
+{
+  return
+    (mSheets && mSheets->mLoadingDatas.Count() != 0) ||
+    (mSheets && mSheets->mPendingDatas.Count() != 0) ||
+    mPostedEvents.Length() != 0 ||
+    mDatasToNotifyOn != 0;
+}
+
+nsresult
+Loader::AddObserver(nsICSSLoaderObserver* aObserver)
+{
+  NS_PRECONDITION(aObserver, "Must have observer");
+  if (mObservers.AppendElementUnlessExists(aObserver)) {
+    return NS_OK;
+  }
+
+  return NS_ERROR_OUT_OF_MEMORY;
+}
+
+void
+Loader::RemoveObserver(nsICSSLoaderObserver* aObserver)
+{
+  mObservers.RemoveElement(aObserver);
+}
+
+void
+Loader::StartAlternateLoads()
+{
+  NS_PRECONDITION(mSheets, "Don't call me!");
+  LoadDataArray arr(mSheets->mPendingDatas.Count());
+  for (auto iter = mSheets->mPendingDatas.Iter(); !iter.Done(); iter.Next()) {
+    arr.AppendElement(iter.Data());
+    iter.Remove();
+  }
+
+  mDatasToNotifyOn += arr.Length();
+  for (uint32_t i = 0; i < arr.Length(); ++i) {
+    --mDatasToNotifyOn;
+    LoadSheet(arr[i], eSheetNeedsParser, false);
+  }
+}
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(Loader)
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(Loader)
+  if (tmp->mSheets) {
+    for (auto iter = tmp->mSheets->mCompleteSheets.Iter();
+         !iter.Done();
+         iter.Next()) {
+      NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "Sheet cache nsCSSLoader");
+      if (iter.UserData()->IsGecko()) {
+        CSSStyleSheet* sheet = iter.UserData()->AsGecko();
+        cb.NoteXPCOMChild(NS_ISUPPORTS_CAST(nsIDOMCSSStyleSheet*, sheet));
+      }
+    }
+  }
+  nsTObserverArray<nsCOMPtr<nsICSSLoaderObserver>>::ForwardIterator
+    it(tmp->mObservers);
+  while (it.HasMore()) {
+    ImplCycleCollectionTraverse(cb, it.GetNext(),
+                                "mozilla::css::Loader.mObservers");
+  }
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(Loader)
+  if (tmp->mSheets) {
+    tmp->mSheets->mCompleteSheets.Clear();
+  }
+  tmp->mObservers.Clear();
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_CYCLE_COLLECTION_ROOT_NATIVE(Loader, AddRef)
+NS_IMPL_CYCLE_COLLECTION_UNROOT_NATIVE(Loader, Release)
+
+size_t
+Loader::SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const
+{
+  size_t n = aMallocSizeOf(this);
+
+  if (mSheets) {
+    n += mSheets->mCompleteSheets.ShallowSizeOfExcludingThis(aMallocSizeOf);
+    for (auto iter = mSheets->mCompleteSheets.ConstIter();
+         !iter.Done();
+         iter.Next()) {
+      // 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 StyleSheet* sheet = iter.UserData();
+      n += (sheet->GetOwnerNode() || sheet->GetParentSheet())
+         ? 0
+         : sheet->SizeOfIncludingThis(aMallocSizeOf);
+    }
+  }
+  n += mObservers.ShallowSizeOfExcludingThis(aMallocSizeOf);
+
+  // 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
+  // - mParsingDatas: 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;
+}
+
+StyleBackendType
+Loader::GetStyleBackendType() const
+{
+  MOZ_ASSERT(mStyleBackendType || mDocument,
+             "you must construct a Loader with a document or set a "
+             "StyleBackendType on it before calling GetStyleBackendType");
+  if (mStyleBackendType) {
+    return *mStyleBackendType;
+  }
+  return mDocument->GetStyleBackendType();
+}
+
+} // namespace css
+} // namespace mozilla