Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

8 files changed, 1523 insertions, 0 deletions

diff --git a/dom/prototype/PrototypeDocumentContentSink.cpp b/dom/prototype/PrototypeDocumentContentSink.cpp
new file mode 100644
index 0000000000..2c299f6a91
--- /dev/null
+++ b/dom/prototype/PrototypeDocumentContentSink.cpp
@@ -0,0 +1,1137 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+#include "nsCOMPtr.h"
+#include "mozilla/dom/PrototypeDocumentContentSink.h"
+#include "nsIParser.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/URL.h"
+#include "nsIContent.h"
+#include "nsIURI.h"
+#include "nsNetUtil.h"
+#include "nsHTMLParts.h"
+#include "nsCRT.h"
+#include "mozilla/StyleSheetInlines.h"
+#include "mozilla/css/Loader.h"
+#include "nsGkAtoms.h"
+#include "nsContentUtils.h"
+#include "nsDocElementCreatedNotificationRunner.h"
+#include "nsIScriptContext.h"
+#include "nsNameSpaceManager.h"
+#include "nsIScriptError.h"
+#include "prtime.h"
+#include "mozilla/Logging.h"
+#include "nsRect.h"
+#include "nsIScriptElement.h"
+#include "nsReadableUtils.h"
+#include "nsUnicharUtils.h"
+#include "nsIChannel.h"
+#include "nsNodeInfoManager.h"
+#include "nsContentCreatorFunctions.h"
+#include "nsIContentPolicy.h"
+#include "nsContentPolicyUtils.h"
+#include "nsError.h"
+#include "nsIScriptGlobalObject.h"
+#include "mozAutoDocUpdate.h"
+#include "nsMimeTypes.h"
+#include "nsHtml5SVGLoadDispatcher.h"
+#include "nsTextNode.h"
+#include "mozilla/dom/AutoEntryScript.h"
+#include "mozilla/dom/CDATASection.h"
+#include "mozilla/dom/Comment.h"
+#include "mozilla/dom/DocumentType.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/HTMLTemplateElement.h"
+#include "mozilla/dom/ProcessingInstruction.h"
+#include "mozilla/dom/XMLStylesheetProcessingInstruction.h"
+#include "mozilla/dom/ScriptLoader.h"
+#include "mozilla/LoadInfo.h"
+#include "mozilla/PresShell.h"
+#include "mozilla/ProfilerLabels.h"
+#include "mozilla/RefPtr.h"
+
+#include "nsXULPrototypeCache.h"
+#include "nsXULElement.h"
+#include "mozilla/CycleCollectedJSContext.h"
+#include "js/CompilationAndEvaluation.h"
+#include "js/experimental/JSStencil.h"
+
+using namespace mozilla;
+using namespace mozilla::dom;
+
+LazyLogModule PrototypeDocumentContentSink::gLog("PrototypeDocument");
+
+nsresult NS_NewPrototypeDocumentContentSink(nsIContentSink** aResult,
+                                            Document* aDoc, nsIURI* aURI,
+                                            nsISupports* aContainer,
+                                            nsIChannel* aChannel) {
+  MOZ_ASSERT(nullptr != aResult, "null ptr");
+  if (nullptr == aResult) {
+    return NS_ERROR_NULL_POINTER;
+  }
+  RefPtr<PrototypeDocumentContentSink> it = new PrototypeDocumentContentSink();
+
+  nsresult rv = it->Init(aDoc, aURI, aContainer, aChannel);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  it.forget(aResult);
+  return NS_OK;
+}
+
+namespace mozilla::dom {
+
+PrototypeDocumentContentSink::PrototypeDocumentContentSink()
+    : mNextSrcLoadWaiter(nullptr),
+      mCurrentScriptProto(nullptr),
+      mOffThreadCompiling(false),
+      mOffThreadCompileStringBuf(nullptr),
+      mOffThreadCompileStringLength(0),
+      mStillWalking(false),
+      mPendingSheets(0) {}
+
+PrototypeDocumentContentSink::~PrototypeDocumentContentSink() {
+  NS_ASSERTION(
+      mNextSrcLoadWaiter == nullptr,
+      "unreferenced document still waiting for script source to load?");
+
+  if (mOffThreadCompileStringBuf) {
+    js_free(mOffThreadCompileStringBuf);
+  }
+}
+
+nsresult PrototypeDocumentContentSink::Init(Document* aDoc, nsIURI* aURI,
+                                            nsISupports* aContainer,
+                                            nsIChannel* aChannel) {
+  MOZ_ASSERT(aDoc, "null ptr");
+  MOZ_ASSERT(aURI, "null ptr");
+
+  mDocument = aDoc;
+
+  mDocument->SetDelayFrameLoaderInitialization(true);
+  mDocument->SetMayStartLayout(false);
+
+  // Get the URI.  this should match the uri used for the OnNewURI call in
+  // nsDocShell::CreateContentViewer.
+  nsresult rv = NS_GetFinalChannelURI(aChannel, getter_AddRefs(mDocumentURI));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  mScriptLoader = mDocument->ScriptLoader();
+
+  return NS_OK;
+}
+
+NS_IMPL_CYCLE_COLLECTION(PrototypeDocumentContentSink, mParser, mDocumentURI,
+                         mDocument, mScriptLoader, mContextStack,
+                         mCurrentPrototype)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(PrototypeDocumentContentSink)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIContentSink)
+  NS_INTERFACE_MAP_ENTRY(nsIContentSink)
+  NS_INTERFACE_MAP_ENTRY(nsIStreamLoaderObserver)
+  NS_INTERFACE_MAP_ENTRY(nsICSSLoaderObserver)
+  NS_INTERFACE_MAP_ENTRY(nsIOffThreadScriptReceiver)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(PrototypeDocumentContentSink)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(PrototypeDocumentContentSink)
+
+//----------------------------------------------------------------------
+//
+// nsIContentSink interface
+//
+
+void PrototypeDocumentContentSink::SetDocumentCharset(
+    NotNull<const Encoding*> aEncoding) {
+  if (mDocument) {
+    mDocument->SetDocumentCharacterSet(aEncoding);
+  }
+}
+
+nsISupports* PrototypeDocumentContentSink::GetTarget() {
+  return ToSupports(mDocument);
+}
+
+bool PrototypeDocumentContentSink::IsScriptExecuting() {
+  return !!mScriptLoader->GetCurrentScript();
+}
+
+NS_IMETHODIMP
+PrototypeDocumentContentSink::SetParser(nsParserBase* aParser) {
+  MOZ_ASSERT(aParser, "Should have a parser here!");
+  mParser = aParser;
+  return NS_OK;
+}
+
+nsIParser* PrototypeDocumentContentSink::GetParser() {
+  return static_cast<nsIParser*>(mParser.get());
+}
+
+void PrototypeDocumentContentSink::ContinueInterruptedParsingIfEnabled() {
+  if (mParser && mParser->IsParserEnabled()) {
+    GetParser()->ContinueInterruptedParsing();
+  }
+}
+
+void PrototypeDocumentContentSink::ContinueInterruptedParsingAsync() {
+  nsCOMPtr<nsIRunnable> ev = NewRunnableMethod(
+      "PrototypeDocumentContentSink::ContinueInterruptedParsingIfEnabled", this,
+      &PrototypeDocumentContentSink::ContinueInterruptedParsingIfEnabled);
+
+  mDocument->Dispatch(mozilla::TaskCategory::Other, ev.forget());
+}
+
+//----------------------------------------------------------------------
+//
+// PrototypeDocumentContentSink::ContextStack
+//
+
+PrototypeDocumentContentSink::ContextStack::ContextStack()
+    : mTop(nullptr), mDepth(0) {}
+
+PrototypeDocumentContentSink::ContextStack::~ContextStack() { Clear(); }
+
+void PrototypeDocumentContentSink::ContextStack::Traverse(
+    nsCycleCollectionTraversalCallback& aCallback, const char* aName,
+    uint32_t aFlags) {
+  aFlags |= CycleCollectionEdgeNameArrayFlag;
+  Entry* current = mTop;
+  while (current) {
+    CycleCollectionNoteChild(aCallback, current->mElement, aName, aFlags);
+    current = current->mNext;
+  }
+}
+
+void PrototypeDocumentContentSink::ContextStack::Clear() {
+  while (mTop) {
+    Entry* doomed = mTop;
+    mTop = mTop->mNext;
+    NS_IF_RELEASE(doomed->mElement);
+    delete doomed;
+  }
+  mDepth = 0;
+}
+
+nsresult PrototypeDocumentContentSink::ContextStack::Push(
+    nsXULPrototypeElement* aPrototype, nsIContent* aElement) {
+  Entry* entry = new Entry;
+  entry->mPrototype = aPrototype;
+  entry->mElement = aElement;
+  NS_IF_ADDREF(entry->mElement);
+  entry->mIndex = 0;
+
+  entry->mNext = mTop;
+  mTop = entry;
+
+  ++mDepth;
+  return NS_OK;
+}
+
+nsresult PrototypeDocumentContentSink::ContextStack::Pop() {
+  if (mDepth == 0) return NS_ERROR_UNEXPECTED;
+
+  Entry* doomed = mTop;
+  mTop = mTop->mNext;
+  --mDepth;
+
+  NS_IF_RELEASE(doomed->mElement);
+  delete doomed;
+  return NS_OK;
+}
+
+nsresult PrototypeDocumentContentSink::ContextStack::Peek(
+    nsXULPrototypeElement** aPrototype, nsIContent** aElement,
+    int32_t* aIndex) {
+  if (mDepth == 0) return NS_ERROR_UNEXPECTED;
+
+  *aPrototype = mTop->mPrototype;
+  *aElement = mTop->mElement;
+  NS_IF_ADDREF(*aElement);
+  *aIndex = mTop->mIndex;
+
+  return NS_OK;
+}
+
+nsresult PrototypeDocumentContentSink::ContextStack::SetTopIndex(
+    int32_t aIndex) {
+  if (mDepth == 0) return NS_ERROR_UNEXPECTED;
+
+  mTop->mIndex = aIndex;
+  return NS_OK;
+}
+
+//----------------------------------------------------------------------
+//
+// Content model walking routines
+//
+
+nsresult PrototypeDocumentContentSink::OnPrototypeLoadDone(
+    nsXULPrototypeDocument* aPrototype) {
+  mCurrentPrototype = aPrototype;
+  mDocument->SetPrototypeDocument(aPrototype);
+
+  nsresult rv = PrepareToWalk();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = ResumeWalk();
+
+  return rv;
+}
+
+nsresult PrototypeDocumentContentSink::PrepareToWalk() {
+  MOZ_ASSERT(mCurrentPrototype);
+  nsresult rv;
+
+  mStillWalking = true;
+
+  // Notify document that the load is beginning
+  mDocument->BeginLoad();
+
+  // Get the prototype's root element and initialize the context
+  // stack for the prototype walk.
+  nsXULPrototypeElement* proto = mCurrentPrototype->GetRootElement();
+
+  if (!proto) {
+    if (MOZ_LOG_TEST(gLog, LogLevel::Error)) {
+      nsCOMPtr<nsIURI> url = mCurrentPrototype->GetURI();
+
+      nsAutoCString urlspec;
+      rv = url->GetSpec(urlspec);
+      if (NS_FAILED(rv)) return rv;
+
+      MOZ_LOG(gLog, LogLevel::Error,
+              ("prototype: error parsing '%s'", urlspec.get()));
+    }
+
+    return NS_OK;
+  }
+
+  nsINode* nodeToInsertBefore = mDocument->GetFirstChild();
+
+  const nsTArray<RefPtr<nsXULPrototypePI> >& processingInstructions =
+      mCurrentPrototype->GetProcessingInstructions();
+
+  uint32_t total = processingInstructions.Length();
+  for (uint32_t i = 0; i < total; ++i) {
+    rv = CreateAndInsertPI(processingInstructions[i], mDocument,
+                           nodeToInsertBefore);
+    if (NS_FAILED(rv)) return rv;
+  }
+
+  // Do one-time initialization.
+  RefPtr<Element> root;
+
+  // Add the root element
+  rv = CreateElementFromPrototype(proto, getter_AddRefs(root), nullptr);
+  if (NS_FAILED(rv)) return rv;
+
+  ErrorResult error;
+  mDocument->AppendChildTo(root, false, error);
+  if (error.Failed()) {
+    return error.StealNSResult();
+  }
+
+  // TODO(emilio): Should this really notify? We don't notify of appends anyhow,
+  // and we just appended the root so no styles can possibly depend on it.
+  mDocument->UpdateDocumentStates(DocumentState::RTL_LOCALE, true);
+
+  nsContentUtils::AddScriptRunner(
+      new nsDocElementCreatedNotificationRunner(mDocument));
+
+  // There'd better not be anything on the context stack at this
+  // point! This is the basis case for our "induction" in
+  // ResumeWalk(), below, which'll assume that there's always a
+  // content element on the context stack if we're in the document.
+  NS_ASSERTION(mContextStack.Depth() == 0,
+               "something's on the context stack already");
+  if (mContextStack.Depth() != 0) return NS_ERROR_UNEXPECTED;
+
+  rv = mContextStack.Push(proto, root);
+  if (NS_FAILED(rv)) return rv;
+
+  return NS_OK;
+}
+
+nsresult PrototypeDocumentContentSink::CreateAndInsertPI(
+    const nsXULPrototypePI* aProtoPI, nsINode* aParent, nsINode* aBeforeThis) {
+  MOZ_ASSERT(aProtoPI, "null ptr");
+  MOZ_ASSERT(aParent, "null ptr");
+
+  RefPtr<ProcessingInstruction> node =
+      NS_NewXMLProcessingInstruction(aParent->OwnerDoc()->NodeInfoManager(),
+                                     aProtoPI->mTarget, aProtoPI->mData);
+
+  nsresult rv;
+  if (aProtoPI->mTarget.EqualsLiteral("xml-stylesheet")) {
+    MOZ_ASSERT(LinkStyle::FromNode(*node),
+               "XML Stylesheet node does not implement LinkStyle!");
+    auto* pi = static_cast<XMLStylesheetProcessingInstruction*>(node.get());
+    rv = InsertXMLStylesheetPI(aProtoPI, aParent, aBeforeThis, pi);
+  } else {
+    // No special processing, just add the PI to the document.
+    ErrorResult error;
+    aParent->InsertChildBefore(node->AsContent(),
+                               aBeforeThis ? aBeforeThis->AsContent() : nullptr,
+                               false, error);
+    rv = error.StealNSResult();
+  }
+
+  return rv;
+}
+
+nsresult PrototypeDocumentContentSink::InsertXMLStylesheetPI(
+    const nsXULPrototypePI* aProtoPI, nsINode* aParent, nsINode* aBeforeThis,
+    XMLStylesheetProcessingInstruction* aPINode) {
+  // We want to be notified when the style sheet finishes loading, so
+  // disable style sheet loading for now.
+  aPINode->SetEnableUpdates(false);
+  aPINode->OverrideBaseURI(mCurrentPrototype->GetURI());
+
+  ErrorResult rv;
+  aParent->InsertChildBefore(
+      aPINode, aBeforeThis ? aBeforeThis->AsContent() : nullptr, false, rv);
+  if (rv.Failed()) {
+    return rv.StealNSResult();
+  }
+
+  aPINode->SetEnableUpdates(true);
+
+  // load the stylesheet if necessary, passing ourselves as
+  // nsICSSObserver
+  auto result = aPINode->UpdateStyleSheet(this);
+  if (result.isErr()) {
+    // Ignore errors from UpdateStyleSheet; we don't want failure to
+    // do that to break the XUL document load.  But do propagate out
+    // NS_ERROR_OUT_OF_MEMORY.
+    if (result.unwrapErr() == NS_ERROR_OUT_OF_MEMORY) {
+      return result.unwrapErr();
+    }
+    return NS_OK;
+  }
+
+  auto update = result.unwrap();
+  if (update.ShouldBlock()) {
+    ++mPendingSheets;
+  }
+
+  return NS_OK;
+}
+
+void PrototypeDocumentContentSink::CloseElement(Element* aElement,
+                                                bool aHadChildren) {
+  if (nsIContent::RequiresDoneAddingChildren(
+          aElement->NodeInfo()->NamespaceID(),
+          aElement->NodeInfo()->NameAtom())) {
+    aElement->DoneAddingChildren(false);
+  }
+
+  if (!aHadChildren) {
+    return;
+  }
+
+  // See bug 370111 and bug 1495946. We don't cache inline styles nor module
+  // scripts in the prototype cache, and we don't notify on node insertion, so
+  // we need to do this for the stylesheet / script to be properly processed.
+  // This kinda sucks, but notifying was a pretty sizeable perf regression so...
+  if (aElement->IsHTMLElement(nsGkAtoms::script) ||
+      aElement->IsSVGElement(nsGkAtoms::script)) {
+    nsCOMPtr<nsIScriptElement> sele = do_QueryInterface(aElement);
+    MOZ_ASSERT(sele, "Node didn't QI to script.");
+    if (sele->GetScriptIsModule()) {
+      DebugOnly<bool> block = sele->AttemptToExecute();
+      MOZ_ASSERT(!block, "<script type=module> shouldn't block the parser");
+    }
+  }
+
+  if (aElement->IsHTMLElement(nsGkAtoms::style) ||
+      aElement->IsSVGElement(nsGkAtoms::style)) {
+    auto* linkStyle = LinkStyle::FromNode(*aElement);
+    NS_ASSERTION(linkStyle,
+                 "<html:style> doesn't implement "
+                 "nsIStyleSheetLinkingElement?");
+    Unused << linkStyle->UpdateStyleSheet(nullptr);
+  }
+}
+
+nsresult PrototypeDocumentContentSink::ResumeWalk() {
+  nsresult rv = ResumeWalkInternal();
+  if (NS_FAILED(rv)) {
+    nsContentUtils::ReportToConsoleNonLocalized(
+        u"Failed to load document from prototype document."_ns,
+        nsIScriptError::errorFlag, "Prototype Document"_ns, mDocument,
+        mDocumentURI);
+  }
+  return rv;
+}
+
+nsresult PrototypeDocumentContentSink::ResumeWalkInternal() {
+  MOZ_ASSERT(mStillWalking);
+  // Walk the prototype and build the delegate content model. The
+  // walk is performed in a top-down, left-to-right fashion. That
+  // is, a parent is built before any of its children; a node is
+  // only built after all of its siblings to the left are fully
+  // constructed.
+  //
+  // It is interruptable so that transcluded documents (e.g.,
+  // <html:script src="..." />) can be properly re-loaded if the
+  // cached copy of the document becomes stale.
+  nsresult rv;
+  nsCOMPtr<nsIURI> docURI =
+      mCurrentPrototype ? mCurrentPrototype->GetURI() : nullptr;
+
+  while (1) {
+    // Begin (or resume) walking the current prototype.
+
+    while (mContextStack.Depth() > 0) {
+      // Look at the top of the stack to determine what we're
+      // currently working on.
+      // This will always be a node already constructed and
+      // inserted to the actual document.
+      nsXULPrototypeElement* proto;
+      nsCOMPtr<nsIContent> element;
+      nsCOMPtr<nsIContent> nodeToPushTo;
+      int32_t indx;  // all children of proto before indx (not
+                     // inclusive) have already been constructed
+      rv = mContextStack.Peek(&proto, getter_AddRefs(element), &indx);
+      if (NS_FAILED(rv)) return rv;
+
+      if (indx >= (int32_t)proto->mChildren.Length()) {
+        if (element) {
+          // We've processed all of the prototype's children.
+          CloseElement(element->AsElement(), /* aHadChildren = */ true);
+        }
+        // Now pop the context stack back up to the parent
+        // element and continue the prototype walk.
+        mContextStack.Pop();
+        continue;
+      }
+
+      nodeToPushTo = element;
+      // For template elements append the content to the template's document
+      // fragment.
+      if (auto* templateElement = HTMLTemplateElement::FromNode(element)) {
+        nodeToPushTo = templateElement->Content();
+      }
+
+      // Grab the next child, and advance the current context stack
+      // to the next sibling to our right.
+      nsXULPrototypeNode* childproto = proto->mChildren[indx];
+      mContextStack.SetTopIndex(++indx);
+
+      switch (childproto->mType) {
+        case nsXULPrototypeNode::eType_Element: {
+          // An 'element', which may contain more content.
+          auto* protoele = static_cast<nsXULPrototypeElement*>(childproto);
+
+          RefPtr<Element> child;
+
+          rv = CreateElementFromPrototype(protoele, getter_AddRefs(child),
+                                          nodeToPushTo);
+          if (NS_FAILED(rv)) return rv;
+
+          // ...and append it to the content model.
+          ErrorResult error;
+          nodeToPushTo->AppendChildTo(child, false, error);
+          if (error.Failed()) {
+            return error.StealNSResult();
+          }
+
+          if (nsIContent::RequiresDoneCreatingElement(
+                  protoele->mNodeInfo->NamespaceID(),
+                  protoele->mNodeInfo->NameAtom())) {
+            child->DoneCreatingElement();
+          }
+
+          // If it has children, push the element onto the context
+          // stack and begin to process them.
+          if (protoele->mChildren.Length() > 0) {
+            rv = mContextStack.Push(protoele, child);
+            if (NS_FAILED(rv)) return rv;
+          } else {
+            // If there are no children, close the element immediately.
+            CloseElement(child, /* aHadChildren = */ false);
+          }
+        } break;
+
+        case nsXULPrototypeNode::eType_Script: {
+          // A script reference. Execute the script immediately;
+          // this may have side effects in the content model.
+          auto* scriptproto = static_cast<nsXULPrototypeScript*>(childproto);
+          if (scriptproto->mSrcURI) {
+            // A transcluded script reference; this may
+            // "block" our prototype walk if the script isn't
+            // cached, or the cached copy of the script is
+            // stale and must be reloaded.
+            bool blocked;
+            rv = LoadScript(scriptproto, &blocked);
+            // If the script cannot be loaded, just keep going!
+
+            if (NS_SUCCEEDED(rv) && blocked) return NS_OK;
+          } else if (scriptproto->HasStencil()) {
+            // An inline script
+            rv = ExecuteScript(scriptproto);
+            if (NS_FAILED(rv)) return rv;
+          }
+        } break;
+
+        case nsXULPrototypeNode::eType_Text: {
+          nsNodeInfoManager* nim = nodeToPushTo->NodeInfo()->NodeInfoManager();
+          // A simple text node.
+          RefPtr<nsTextNode> text = new (nim) nsTextNode(nim);
+
+          auto* textproto = static_cast<nsXULPrototypeText*>(childproto);
+          text->SetText(textproto->mValue, false);
+
+          ErrorResult error;
+          nodeToPushTo->AppendChildTo(text, false, error);
+          if (error.Failed()) {
+            return error.StealNSResult();
+          }
+        } break;
+
+        case nsXULPrototypeNode::eType_PI: {
+          auto* piProto = static_cast<nsXULPrototypePI*>(childproto);
+
+          // <?xml-stylesheet?> doesn't have an effect
+          // outside the prolog, like it used to. Issue a warning.
+
+          if (piProto->mTarget.EqualsLiteral("xml-stylesheet")) {
+            AutoTArray<nsString, 1> params = {piProto->mTarget};
+
+            nsContentUtils::ReportToConsole(nsIScriptError::warningFlag,
+                                            "XUL Document"_ns, nullptr,
+                                            nsContentUtils::eXUL_PROPERTIES,
+                                            "PINotInProlog", params, docURI);
+          }
+
+          nsIContent* parent = element.get();
+          if (parent) {
+            // an inline script could have removed the root element
+            rv = CreateAndInsertPI(piProto, parent, nullptr);
+            NS_ENSURE_SUCCESS(rv, rv);
+          }
+        } break;
+
+        default:
+          MOZ_ASSERT_UNREACHABLE("Unexpected nsXULPrototypeNode::Type");
+      }
+    }
+
+    // Once we get here, the context stack will have been
+    // depleted. That means that the entire prototype has been
+    // walked and content has been constructed.
+    break;
+  }
+
+  mStillWalking = false;
+  return MaybeDoneWalking();
+}
+
+void PrototypeDocumentContentSink::InitialTranslationCompleted() {
+  MaybeDoneWalking();
+}
+
+nsresult PrototypeDocumentContentSink::MaybeDoneWalking() {
+  if (mPendingSheets > 0 || mStillWalking) {
+    return NS_OK;
+  }
+
+  if (mDocument->HasPendingInitialTranslation()) {
+    mDocument->OnParsingCompleted();
+    return NS_OK;
+  }
+
+  return DoneWalking();
+}
+
+nsresult PrototypeDocumentContentSink::DoneWalking() {
+  MOZ_ASSERT(mPendingSheets == 0, "there are sheets to be loaded");
+  MOZ_ASSERT(!mStillWalking, "walk not done");
+  MOZ_ASSERT(!mDocument->HasPendingInitialTranslation(), "translation pending");
+
+  if (mDocument) {
+    MOZ_ASSERT(mDocument->GetReadyStateEnum() == Document::READYSTATE_LOADING,
+               "Bad readyState");
+    mDocument->SetReadyStateInternal(Document::READYSTATE_INTERACTIVE);
+    mDocument->NotifyPossibleTitleChange(false);
+
+    nsContentUtils::DispatchEventOnlyToChrome(mDocument, ToSupports(mDocument),
+                                              u"MozBeforeInitialXULLayout"_ns,
+                                              CanBubble::eYes, Cancelable::eNo);
+  }
+
+  if (mScriptLoader) {
+    mScriptLoader->ParsingComplete(false);
+    mScriptLoader->DeferCheckpointReached();
+  }
+
+  StartLayout();
+
+  if (IsChromeURI(mDocumentURI) &&
+      nsXULPrototypeCache::GetInstance()->IsEnabled()) {
+    bool isCachedOnDisk;
+    nsXULPrototypeCache::GetInstance()->HasPrototype(mDocumentURI,
+                                                     &isCachedOnDisk);
+    if (!isCachedOnDisk) {
+      nsXULPrototypeCache::GetInstance()->WritePrototype(mCurrentPrototype);
+    }
+  }
+
+  mDocument->SetDelayFrameLoaderInitialization(false);
+  RefPtr<Document> doc = mDocument;
+  doc->MaybeInitializeFinalizeFrameLoaders();
+
+  // If the document we are loading has a reference or it is a
+  // frameset document, disable the scroll bars on the views.
+
+  doc->SetScrollToRef(mDocument->GetDocumentURI());
+
+  doc->EndLoad();
+
+  return NS_OK;
+}
+
+void PrototypeDocumentContentSink::StartLayout() {
+  AUTO_PROFILER_LABEL_DYNAMIC_NSCSTRING(
+      "PrototypeDocumentContentSink::StartLayout", LAYOUT,
+      mDocumentURI->GetSpecOrDefault());
+  mDocument->SetMayStartLayout(true);
+  RefPtr<PresShell> presShell = mDocument->GetPresShell();
+  if (presShell && !presShell->DidInitialize()) {
+    nsresult rv = presShell->Initialize();
+    if (NS_FAILED(rv)) {
+      return;
+    }
+  }
+}
+
+NS_IMETHODIMP
+PrototypeDocumentContentSink::StyleSheetLoaded(StyleSheet* aSheet,
+                                               bool aWasDeferred,
+                                               nsresult aStatus) {
+  if (!aWasDeferred) {
+    // Don't care about when alternate sheets finish loading
+    MOZ_ASSERT(mPendingSheets > 0, "Unexpected StyleSheetLoaded notification");
+
+    --mPendingSheets;
+
+    return MaybeDoneWalking();
+  }
+
+  return NS_OK;
+}
+
+nsresult PrototypeDocumentContentSink::LoadScript(
+    nsXULPrototypeScript* aScriptProto, bool* aBlock) {
+  // Load a transcluded script
+  nsresult rv;
+
+  bool isChromeDoc = IsChromeURI(mDocumentURI);
+
+  if (isChromeDoc && aScriptProto->HasStencil()) {
+    rv = ExecuteScript(aScriptProto);
+
+    // Ignore return value from execution, and don't block
+    *aBlock = false;
+    return NS_OK;
+  }
+
+  // Try the XUL script cache, in case two XUL documents source the same
+  // .js file (e.g., strres.js from navigator.xul and utilityOverlay.xul).
+  // XXXbe the cache relies on aScriptProto's GC root!
+  bool useXULCache = nsXULPrototypeCache::GetInstance()->IsEnabled();
+
+  if (isChromeDoc && useXULCache) {
+    RefPtr<JS::Stencil> newStencil =
+        nsXULPrototypeCache::GetInstance()->GetStencil(aScriptProto->mSrcURI);
+    if (newStencil) {
+      // The script language for a proto must remain constant - we
+      // can't just change it for this unexpected language.
+      aScriptProto->Set(newStencil);
+    }
+
+    if (aScriptProto->HasStencil()) {
+      rv = ExecuteScript(aScriptProto);
+
+      // Ignore return value from execution, and don't block
+      *aBlock = false;
+      return NS_OK;
+    }
+  }
+
+  // Release stencil from FastLoad since we decided against using them
+  aScriptProto->Set(nullptr);
+
+  // Set the current script prototype so that OnStreamComplete can report
+  // the right file if there are errors in the script.
+  NS_ASSERTION(!mCurrentScriptProto,
+               "still loading a script when starting another load?");
+  mCurrentScriptProto = aScriptProto;
+
+  if (isChromeDoc && aScriptProto->mSrcLoading) {
+    // Another document load has started, which is still in progress.
+    // Remember to ResumeWalk this document when the load completes.
+    mNextSrcLoadWaiter = aScriptProto->mSrcLoadWaiters;
+    aScriptProto->mSrcLoadWaiters = this;
+    NS_ADDREF_THIS();
+  } else {
+    nsCOMPtr<nsILoadGroup> group =
+        mDocument
+            ->GetDocumentLoadGroup();  // found in
+                                       // mozilla::dom::Document::SetScriptGlobalObject
+
+    // Note: the loader will keep itself alive while it's loading.
+    nsCOMPtr<nsIStreamLoader> loader;
+    rv = NS_NewStreamLoader(
+        getter_AddRefs(loader), aScriptProto->mSrcURI,
+        this,       // aObserver
+        mDocument,  // aRequestingContext
+        nsILoadInfo::SEC_REQUIRE_SAME_ORIGIN_INHERITS_SEC_CONTEXT,
+        nsIContentPolicy::TYPE_INTERNAL_SCRIPT, group);
+
+    if (NS_FAILED(rv)) {
+      mCurrentScriptProto = nullptr;
+      return rv;
+    }
+
+    aScriptProto->mSrcLoading = true;
+  }
+
+  // Block until OnStreamComplete resumes us.
+  *aBlock = true;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+PrototypeDocumentContentSink::OnStreamComplete(nsIStreamLoader* aLoader,
+                                               nsISupports* context,
+                                               nsresult aStatus,
+                                               uint32_t stringLen,
+                                               const uint8_t* string) {
+  nsCOMPtr<nsIRequest> request;
+  aLoader->GetRequest(getter_AddRefs(request));
+  nsCOMPtr<nsIChannel> channel = do_QueryInterface(request);
+
+#ifdef DEBUG
+  // print a load error on bad status
+  if (NS_FAILED(aStatus)) {
+    if (channel) {
+      nsCOMPtr<nsIURI> uri;
+      channel->GetURI(getter_AddRefs(uri));
+      if (uri) {
+        printf("Failed to load %s\n", uri->GetSpecOrDefault().get());
+      }
+    }
+  }
+#endif
+
+  // This is the completion routine that will be called when a
+  // transcluded script completes. Compile and execute the script
+  // if the load was successful, then continue building content
+  // from the prototype.
+  nsresult rv = aStatus;
+
+  NS_ASSERTION(mCurrentScriptProto && mCurrentScriptProto->mSrcLoading,
+               "script source not loading on unichar stream complete?");
+  if (!mCurrentScriptProto) {
+    // XXX Wallpaper for bug 270042
+    return NS_OK;
+  }
+
+  if (NS_SUCCEEDED(aStatus)) {
+    // If the including document is a FastLoad document, and we're
+    // compiling an out-of-line script (one with src=...), then we must
+    // be writing a new FastLoad file.  If we were reading this script
+    // from the FastLoad file, XULContentSinkImpl::OpenScript (over in
+    // nsXULContentSink.cpp) would have already deserialized a non-null
+    // script->mStencil, causing control flow at the top of LoadScript
+    // not to reach here.
+    nsCOMPtr<nsIURI> uri = mCurrentScriptProto->mSrcURI;
+
+    // XXX should also check nsIHttpChannel::requestSucceeded
+
+    MOZ_ASSERT(!mOffThreadCompiling && (mOffThreadCompileStringLength == 0 &&
+                                        !mOffThreadCompileStringBuf),
+               "PrototypeDocument can't load multiple scripts at once");
+
+    rv = ScriptLoader::ConvertToUTF8(channel, string, stringLen, u""_ns,
+                                     mDocument, mOffThreadCompileStringBuf,
+                                     mOffThreadCompileStringLength);
+    if (NS_SUCCEEDED(rv)) {
+      // Pass ownership of the buffer, carefully emptying the existing
+      // fields in the process.  Note that the |Compile| function called
+      // below always takes ownership of the buffer.
+      Utf8Unit* units = nullptr;
+      size_t unitsLength = 0;
+
+      std::swap(units, mOffThreadCompileStringBuf);
+      std::swap(unitsLength, mOffThreadCompileStringLength);
+
+      rv = mCurrentScriptProto->Compile(units, unitsLength,
+                                        JS::SourceOwnership::TakeOwnership, uri,
+                                        1, mDocument, this);
+      if (NS_SUCCEEDED(rv) && !mCurrentScriptProto->HasStencil()) {
+        mOffThreadCompiling = true;
+        mDocument->BlockOnload();
+        return NS_OK;
+      }
+    }
+  }
+
+  return OnScriptCompileComplete(mCurrentScriptProto->GetStencil(), rv);
+}
+
+NS_IMETHODIMP
+PrototypeDocumentContentSink::OnScriptCompileComplete(JS::Stencil* aStencil,
+                                                      nsresult aStatus) {
+  // The mCurrentScriptProto may have been cleared out by another
+  // PrototypeDocumentContentSink.
+  if (!mCurrentScriptProto) {
+    return NS_OK;
+  }
+
+  // When compiling off thread the script will not have been attached to the
+  // script proto yet.
+  if (aStencil && !mCurrentScriptProto->HasStencil()) {
+    mCurrentScriptProto->Set(aStencil);
+  }
+
+  // Allow load events to be fired once off thread compilation finishes.
+  if (mOffThreadCompiling) {
+    mOffThreadCompiling = false;
+    mDocument->UnblockOnload(false);
+  }
+
+  // After compilation finishes the script's characters are no longer needed.
+  if (mOffThreadCompileStringBuf) {
+    js_free(mOffThreadCompileStringBuf);
+    mOffThreadCompileStringBuf = nullptr;
+    mOffThreadCompileStringLength = 0;
+  }
+
+  // Clear mCurrentScriptProto now, but save it first for use below in
+  // the execute code, and in the while loop that resumes walks of other
+  // documents that raced to load this script.
+  nsXULPrototypeScript* scriptProto = mCurrentScriptProto;
+  mCurrentScriptProto = nullptr;
+
+  // Clear the prototype's loading flag before executing the script or
+  // resuming document walks, in case any of those control flows starts a
+  // new script load.
+  scriptProto->mSrcLoading = false;
+
+  nsresult rv = aStatus;
+  if (NS_SUCCEEDED(rv)) {
+    rv = ExecuteScript(scriptProto);
+
+    // If the XUL cache is enabled, save the script object there in
+    // case different XUL documents source the same script.
+    //
+    // But don't save the script in the cache unless the master XUL
+    // document URL is a chrome: URL.  It is valid for a URL such as
+    // about:config to translate into a master document URL, whose
+    // prototype document nodes -- including prototype scripts that
+    // hold GC roots protecting their mJSObject pointers -- are not
+    // cached in the XUL prototype cache.  See StartDocumentLoad,
+    // the fillXULCache logic.
+    //
+    // A document such as about:config is free to load a script via
+    // a URL such as chrome://global/content/config.js, and we must
+    // not cache that script object without a prototype cache entry
+    // containing a companion nsXULPrototypeScript node that owns a
+    // GC root protecting the script object.  Otherwise, the script
+    // cache entry will dangle once the uncached prototype document
+    // is released when its owning document is unloaded.
+    //
+    // (See http://bugzilla.mozilla.org/show_bug.cgi?id=98207 for
+    // the true crime story.)
+    bool useXULCache = nsXULPrototypeCache::GetInstance()->IsEnabled();
+
+    if (useXULCache && IsChromeURI(mDocumentURI) && scriptProto->HasStencil()) {
+      nsXULPrototypeCache::GetInstance()->PutStencil(scriptProto->mSrcURI,
+                                                     scriptProto->GetStencil());
+    }
+    // ignore any evaluation errors
+  }
+
+  rv = ResumeWalk();
+
+  // Load a pointer to the prototype-script's list of documents who
+  // raced to load the same script
+  PrototypeDocumentContentSink** docp = &scriptProto->mSrcLoadWaiters;
+
+  // Resume walking other documents that waited for this one's load, first
+  // executing the script we just compiled, in each doc's script context
+  PrototypeDocumentContentSink* doc;
+  while ((doc = *docp) != nullptr) {
+    NS_ASSERTION(doc->mCurrentScriptProto == scriptProto,
+                 "waiting for wrong script to load?");
+    doc->mCurrentScriptProto = nullptr;
+
+    // Unlink doc from scriptProto's list before executing and resuming
+    *docp = doc->mNextSrcLoadWaiter;
+    doc->mNextSrcLoadWaiter = nullptr;
+
+    if (aStatus == NS_BINDING_ABORTED && !scriptProto->HasStencil()) {
+      // If the previous doc load was aborted, we want to try loading
+      // again for the next doc. Otherwise, one abort would lead to all
+      // subsequent waiting docs to abort as well.
+      bool block = false;
+      doc->LoadScript(scriptProto, &block);
+      NS_RELEASE(doc);
+      return rv;
+    }
+
+    // Execute only if we loaded and compiled successfully, then resume
+    if (NS_SUCCEEDED(aStatus) && scriptProto->HasStencil()) {
+      doc->ExecuteScript(scriptProto);
+    }
+    doc->ResumeWalk();
+    NS_RELEASE(doc);
+  }
+
+  return rv;
+}
+
+nsresult PrototypeDocumentContentSink::ExecuteScript(
+    nsXULPrototypeScript* aScript) {
+  MOZ_ASSERT(aScript != nullptr, "null ptr");
+  NS_ENSURE_TRUE(aScript, NS_ERROR_NULL_POINTER);
+
+  nsIScriptGlobalObject* scriptGlobalObject;
+  bool aHasHadScriptHandlingObject;
+  scriptGlobalObject =
+      mDocument->GetScriptHandlingObject(aHasHadScriptHandlingObject);
+
+  NS_ENSURE_TRUE(scriptGlobalObject, NS_ERROR_NOT_INITIALIZED);
+
+  nsresult rv;
+  rv = scriptGlobalObject->EnsureScriptEnvironment();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Execute the precompiled script with the given version
+  nsAutoMicroTask mt;
+
+  // We're about to run script via JS_ExecuteScript, so we need an
+  // AutoEntryScript. This is Gecko specific and not in any spec.
+  AutoEntryScript aes(scriptGlobalObject, "precompiled XUL <script> element");
+  JSContext* cx = aes.cx();
+
+  JS::Rooted<JSScript*> scriptObject(cx);
+  rv = aScript->InstantiateScript(cx, &scriptObject);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  JS::Rooted<JSObject*> global(cx, JS::CurrentGlobalOrNull(cx));
+  NS_ENSURE_TRUE(xpc::Scriptability::Get(global).Allowed(), NS_OK);
+
+  // On failure, ~AutoScriptEntry will handle exceptions, so
+  // there is no need to manually check the return value.
+  JS::Rooted<JS::Value> rval(cx);
+  Unused << JS_ExecuteScript(cx, scriptObject, &rval);
+
+  return NS_OK;
+}
+
+nsresult PrototypeDocumentContentSink::CreateElementFromPrototype(
+    nsXULPrototypeElement* aPrototype, Element** aResult, nsIContent* aParent) {
+  // Create a content model element from a prototype element.
+  MOZ_ASSERT(aPrototype, "null ptr");
+  if (!aPrototype) return NS_ERROR_NULL_POINTER;
+
+  *aResult = nullptr;
+  nsresult rv = NS_OK;
+
+  if (MOZ_LOG_TEST(gLog, LogLevel::Debug)) {
+    MOZ_LOG(
+        gLog, LogLevel::Debug,
+        ("prototype: creating <%s> from prototype",
+         NS_ConvertUTF16toUTF8(aPrototype->mNodeInfo->QualifiedName()).get()));
+  }
+
+  RefPtr<Element> result;
+
+  Document* doc = aParent ? aParent->OwnerDoc() : mDocument.get();
+  if (aPrototype->mNodeInfo->NamespaceEquals(kNameSpaceID_XUL)) {
+    const bool isRoot = !aParent;
+    // If it's a XUL element, it'll be lightweight until somebody
+    // monkeys with it.
+    rv = nsXULElement::CreateFromPrototype(aPrototype, doc, true, isRoot,
+                                           getter_AddRefs(result));
+    if (NS_FAILED(rv)) return rv;
+  } else {
+    // If it's not a XUL element, it's gonna be heavyweight no matter
+    // what. So we need to copy everything out of the prototype
+    // into the element.  Get a nodeinfo from our nodeinfo manager
+    // for this node.
+    RefPtr<NodeInfo> newNodeInfo = doc->NodeInfoManager()->GetNodeInfo(
+        aPrototype->mNodeInfo->NameAtom(),
+        aPrototype->mNodeInfo->GetPrefixAtom(),
+        aPrototype->mNodeInfo->NamespaceID(), nsINode::ELEMENT_NODE);
+    if (!newNodeInfo) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+    const bool isScript =
+        newNodeInfo->Equals(nsGkAtoms::script, kNameSpaceID_XHTML) ||
+        newNodeInfo->Equals(nsGkAtoms::script, kNameSpaceID_SVG);
+    if (aPrototype->mIsAtom &&
+        newNodeInfo->NamespaceID() == kNameSpaceID_XHTML) {
+      rv = NS_NewHTMLElement(getter_AddRefs(result), newNodeInfo.forget(),
+                             NOT_FROM_PARSER, aPrototype->mIsAtom);
+    } else {
+      rv = NS_NewElement(getter_AddRefs(result), newNodeInfo.forget(),
+                         NOT_FROM_PARSER);
+    }
+    if (NS_FAILED(rv)) return rv;
+
+    rv = AddAttributes(aPrototype, result);
+    if (NS_FAILED(rv)) return rv;
+
+    if (isScript) {
+      nsCOMPtr<nsIScriptElement> sele = do_QueryInterface(result);
+      MOZ_ASSERT(sele, "Node didn't QI to script.");
+
+      sele->FreezeExecutionAttrs(doc);
+      // Script loading is handled by the this content sink, so prevent the
+      // script from loading when it is bound to the document.
+      //
+      // NOTE(emilio): This is only done for non-module scripts, because we
+      // don't support caching modules properly yet, see the comment in
+      // XULContentSinkImpl::OpenScript. For non-inline scripts, this is enough,
+      // since we can start the load when the node is inserted. Non-inline
+      // scripts need another special-case in CloseElement.
+      if (!sele->GetScriptIsModule()) {
+        sele->PreventExecution();
+      }
+    }
+  }
+
+  // FIXME(bug 1627474): Is this right if this is inside an <html:template>?
+  if (result->HasAttr(kNameSpaceID_None, nsGkAtoms::datal10nid)) {
+    mDocument->mL10nProtoElements.InsertOrUpdate(result, RefPtr{aPrototype});
+    result->SetElementCreatedFromPrototypeAndHasUnmodifiedL10n();
+  }
+  result.forget(aResult);
+  return NS_OK;
+}
+
+nsresult PrototypeDocumentContentSink::AddAttributes(
+    nsXULPrototypeElement* aPrototype, Element* aElement) {
+  nsresult rv;
+
+  for (size_t i = 0; i < aPrototype->mAttributes.Length(); ++i) {
+    nsXULPrototypeAttribute* protoattr = &(aPrototype->mAttributes[i]);
+    nsAutoString valueStr;
+    protoattr->mValue.ToString(valueStr);
+
+    rv = aElement->SetAttr(protoattr->mName.NamespaceID(),
+                           protoattr->mName.LocalName(),
+                           protoattr->mName.GetPrefix(), valueStr, false);
+    if (NS_FAILED(rv)) return rv;
+  }
+
+  return NS_OK;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/prototype/PrototypeDocumentContentSink.h b/dom/prototype/PrototypeDocumentContentSink.h
new file mode 100644
index 0000000000..c4735ed52c
--- /dev/null
+++ b/dom/prototype/PrototypeDocumentContentSink.h
@@ -0,0 +1,262 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_PrototypeDocumentContentSink_h__
+#define mozilla_dom_PrototypeDocumentContentSink_h__
+
+#include "mozilla/Attributes.h"
+#include "nsIContentSink.h"
+#include "nsTArray.h"
+#include "nsCOMPtr.h"
+#include "nsCRT.h"
+#include "nsCycleCollectionNoteChild.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsIDTD.h"
+#include "mozilla/dom/FromParser.h"
+#include "nsXULPrototypeDocument.h"
+#include "nsIStreamLoader.h"
+#include "nsIScriptContext.h"
+#include "nsICSSLoaderObserver.h"
+#include "mozilla/Logging.h"
+#include "js/experimental/JSStencil.h"
+#include "mozilla/RefPtr.h"
+
+class nsIURI;
+class nsIChannel;
+class nsIContent;
+class nsIParser;
+class nsTextNode;
+class nsINode;
+class nsXULPrototypeElement;
+class nsXULPrototypePI;
+class nsXULPrototypeScript;
+
+namespace mozilla::dom {
+class Element;
+class ScriptLoader;
+class Document;
+class XMLStylesheetProcessingInstruction;
+}  // namespace mozilla::dom
+
+nsresult NS_NewPrototypeDocumentContentSink(nsIContentSink** aResult,
+                                            mozilla::dom::Document* aDoc,
+                                            nsIURI* aURI,
+                                            nsISupports* aContainer,
+                                            nsIChannel* aChannel);
+
+namespace mozilla::dom {
+
+class PrototypeDocumentContentSink final : public nsIStreamLoaderObserver,
+                                           public nsIContentSink,
+                                           public nsICSSLoaderObserver,
+                                           public nsIOffThreadScriptReceiver {
+ public:
+  PrototypeDocumentContentSink();
+
+  nsresult Init(Document* aDoc, nsIURI* aURL, nsISupports* aContainer,
+                nsIChannel* aChannel);
+
+  // nsISupports
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_NSISTREAMLOADEROBSERVER
+
+  NS_DECL_CYCLE_COLLECTION_CLASS_AMBIGUOUS(PrototypeDocumentContentSink,
+                                           nsIContentSink)
+
+  // nsIContentSink
+  NS_IMETHOD WillParse(void) override { return NS_OK; };
+  NS_IMETHOD WillInterrupt(void) override { return NS_OK; };
+  void WillResume() override{};
+  NS_IMETHOD SetParser(nsParserBase* aParser) override;
+  virtual void InitialTranslationCompleted() override;
+  virtual void FlushPendingNotifications(FlushType aType) override{};
+  virtual void SetDocumentCharset(NotNull<const Encoding*> aEncoding) override;
+  virtual nsISupports* GetTarget() override;
+  virtual bool IsScriptExecuting() override;
+  virtual void ContinueInterruptedParsingAsync() override;
+
+  // nsICSSLoaderObserver
+  NS_IMETHOD StyleSheetLoaded(StyleSheet* aSheet, bool aWasDeferred,
+                              nsresult aStatus) override;
+
+  // nsIOffThreadScriptReceiver
+  NS_IMETHOD OnScriptCompileComplete(JS::Stencil* aStencil,
+                                     nsresult aStatus) override;
+
+  nsresult OnPrototypeLoadDone(nsXULPrototypeDocument* aPrototype);
+
+ protected:
+  virtual ~PrototypeDocumentContentSink();
+
+  static LazyLogModule gLog;
+
+  nsIParser* GetParser();
+
+  void ContinueInterruptedParsingIfEnabled();
+  void StartLayout();
+
+  virtual nsresult AddAttributes(nsXULPrototypeElement* aPrototype,
+                                 Element* aElement);
+
+  RefPtr<nsParserBase> mParser;
+  nsCOMPtr<nsIURI> mDocumentURI;
+  RefPtr<Document> mDocument;
+  RefPtr<ScriptLoader> mScriptLoader;
+
+  PrototypeDocumentContentSink* mNextSrcLoadWaiter;  // [OWNER] but not COMPtr
+
+  /**
+   * The prototype-script of the current transcluded script that is being
+   * loaded.  For document.write('<script src="nestedwrite.js"><\/script>')
+   * to work, these need to be in a stack element type, and we need to hold
+   * the top of stack here.
+   */
+  nsXULPrototypeScript* mCurrentScriptProto;
+
+  /**
+   * Whether the current transcluded script is being compiled off thread.
+   * The load event is blocked while this is in progress.
+   */
+  bool mOffThreadCompiling;
+
+  /**
+   * If the current transcluded script is being compiled off thread, the
+   * source for that script.
+   */
+  Utf8Unit* mOffThreadCompileStringBuf;
+  size_t mOffThreadCompileStringLength;
+
+  /**
+   * Wether the prototype document is still be traversed to create the DOM.
+   * Layout will not be started until false.
+   */
+  bool mStillWalking;
+
+  /**
+   * Number of style sheets still loading. Layout will not start until zero.
+   */
+  uint32_t mPendingSheets;
+
+  /**
+   * Context stack, which maintains the state of the Builder and allows
+   * it to be interrupted.
+   */
+  class ContextStack {
+   protected:
+    struct Entry {
+      nsXULPrototypeElement* mPrototype;
+      nsIContent* mElement;
+      int32_t mIndex;
+      Entry* mNext;
+    };
+
+    Entry* mTop;
+    int32_t mDepth;
+
+   public:
+    ContextStack();
+    ~ContextStack();
+
+    int32_t Depth() { return mDepth; }
+
+    nsresult Push(nsXULPrototypeElement* aPrototype, nsIContent* aElement);
+    nsresult Pop();
+    nsresult Peek(nsXULPrototypeElement** aPrototype, nsIContent** aElement,
+                  int32_t* aIndex);
+
+    nsresult SetTopIndex(int32_t aIndex);
+
+    void Traverse(nsCycleCollectionTraversalCallback& aCallback,
+                  const char* aName, uint32_t aFlags = 0);
+    void Clear();
+
+    // Cycle collector helpers for ContextStack.
+    friend void ImplCycleCollectionUnlink(
+        PrototypeDocumentContentSink::ContextStack& aField) {
+      aField.Clear();
+    }
+
+    friend void ImplCycleCollectionTraverse(
+        nsCycleCollectionTraversalCallback& aCallback,
+        PrototypeDocumentContentSink::ContextStack& aField, const char* aName,
+        uint32_t aFlags = 0) {
+      aField.Traverse(aCallback, aName, aFlags);
+    }
+  };
+
+  friend class ContextStack;
+  ContextStack mContextStack;
+
+  /**
+   * The current prototype that we are walking to construct the
+   * content model.
+   */
+  RefPtr<nsXULPrototypeDocument> mCurrentPrototype;
+  nsresult CreateAndInsertPI(const nsXULPrototypePI* aProtoPI);
+  nsresult ExecuteScript(nsXULPrototypeScript* aScript);
+  nsresult LoadScript(nsXULPrototypeScript* aScriptProto, bool* aBlock);
+
+  /**
+   * A wrapper around ResumeWalkInternal to report walking errors.
+   */
+  nsresult ResumeWalk();
+
+  /**
+   * Resume (or initiate) an interrupted (or newly prepared)
+   * prototype walk.
+   */
+  nsresult ResumeWalkInternal();
+
+  /**
+   * Called at the end of ResumeWalk(), from StyleSheetLoaded(),
+   * and from DocumentL10n.
+   * If walking, stylesheets and l10n are not blocking, it
+   * will trigger `DoneWalking()`.
+   */
+  nsresult MaybeDoneWalking();
+
+  /**
+   * Called from `MaybeDoneWalking()`.
+   * Expects that both the prototype document walk is complete and
+   * all referenced stylesheets finished loading.
+   */
+  nsresult DoneWalking();
+
+  /**
+   * Create a delegate content model element from a prototype.
+   * Note that the resulting content node is not bound to any tree
+   */
+  nsresult CreateElementFromPrototype(nsXULPrototypeElement* aPrototype,
+                                      Element** aResult, nsIContent* aParent);
+  /**
+   * Prepare to walk the current prototype.
+   */
+  nsresult PrepareToWalk();
+  /**
+   * Creates a processing instruction based on aProtoPI and inserts
+   * it to the DOM.
+   */
+  nsresult CreateAndInsertPI(const nsXULPrototypePI* aProtoPI, nsINode* aParent,
+                             nsINode* aBeforeThis);
+
+  /**
+   * Inserts the passed <?xml-stylesheet ?> PI at the specified
+   * index. Loads and applies the associated stylesheet
+   * asynchronously.
+   * The prototype document walk can happen before the stylesheets
+   * are loaded, but the final steps in the load process (see
+   * DoneWalking()) are not run before all the stylesheets are done
+   * loading.
+   */
+  nsresult InsertXMLStylesheetPI(const nsXULPrototypePI* aProtoPI,
+                                 nsINode* aParent, nsINode* aBeforeThis,
+                                 XMLStylesheetProcessingInstruction* aPINode);
+  void CloseElement(Element* aElement, bool aHadChildren);
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_PrototypeDocumentContentSink_h__
diff --git a/dom/prototype/moz.build b/dom/prototype/moz.build
new file mode 100644
index 0000000000..f73b3824ac
--- /dev/null
+++ b/dom/prototype/moz.build
@@ -0,0 +1,25 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# 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/.
+
+with Files("**"):
+    BUG_COMPONENT = ("Core", "XUL")
+
+EXPORTS.mozilla.dom += [
+    "PrototypeDocumentContentSink.h",
+]
+
+SOURCES += [
+    "PrototypeDocumentContentSink.cpp",
+]
+
+LOCAL_INCLUDES += [
+    "/dom/base",
+    "/dom/xul",
+]
+
+MOCHITEST_CHROME_MANIFESTS += ["tests/chrome/chrome.ini"]
+
+FINAL_LIBRARY = "xul"
diff --git a/dom/prototype/tests/chrome/chrome.ini b/dom/prototype/tests/chrome/chrome.ini
new file mode 100644
index 0000000000..8de4e4bfb5
--- /dev/null
+++ b/dom/prototype/tests/chrome/chrome.ini
@@ -0,0 +1,7 @@
+[DEFAULT]
+support-files =
+  whitespace.xhtml
+  no_whitespace.xhtml
+  form.xhtml
+
+[test_prototype_document.xhtml]
diff --git a/dom/prototype/tests/chrome/form.xhtml b/dom/prototype/tests/chrome/form.xhtml
new file mode 100644
index 0000000000..0659215e6e
--- /dev/null
+++ b/dom/prototype/tests/chrome/form.xhtml
@@ -0,0 +1,8 @@
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
+"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
+
+<html xmlns="http://www.w3.org/1999/xhtml">
+  <body>
+    <input type="text" id="input" value=""/>
+  </body>
+</html>
diff --git a/dom/prototype/tests/chrome/no_whitespace.xhtml b/dom/prototype/tests/chrome/no_whitespace.xhtml
new file mode 100644
index 0000000000..78a8c34cff
--- /dev/null
+++ b/dom/prototype/tests/chrome/no_whitespace.xhtml
@@ -0,0 +1,3 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/strict.dtd">
+<html xmlns="http://www.w3.org/TR/xhtml1/strict"><body>Hello<p>there!</p></body></html>
diff --git a/dom/prototype/tests/chrome/test_prototype_document.xhtml b/dom/prototype/tests/chrome/test_prototype_document.xhtml
new file mode 100644
index 0000000000..47dff2d565
--- /dev/null
+++ b/dom/prototype/tests/chrome/test_prototype_document.xhtml
@@ -0,0 +1,73 @@
+<?xml version="1.0"?>
+<?xml-stylesheet type="text/css" href="chrome://global/skin"?>
+<?xml-stylesheet type="text/css" href="chrome://mochikit/content/tests/SimpleTest/test.css"?>
+<window title="Test prototype document"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+  <script type="application/javascript"
+          src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <script><![CDATA[
+    SimpleTest.waitForExplicitFinish();
+  ]]></script>
+  <browser type="chrome" id="browser" flex="1"/>
+  <body xmlns="http://www.w3.org/1999/xhtml">
+<p id="display"></p>
+<div id="content" style="display: none">
+
+</div>
+<pre id="test">
+</pre>
+
+
+<script><![CDATA[
+
+async function load(frame, url) {
+  return new Promise((resolve) => {
+    frame.addEventListener("load", () => {
+      resolve();
+    }, {once: true});
+
+    if (frame.src === url) {
+      frame.reload();
+    } else {
+      frame.src = url;
+    }
+  });
+}
+
+// Load a file with and without the prototype document cache enabled.
+async function compare(filename) {
+  let browser = document.getElementById("browser");
+  // Load the page with no prototype document cache (the regular loading flow of
+  // an XHTML page).
+  await SpecialPowers.pushPrefEnv({ set: [["dom.prototype_document_cache.enabled", false]] });
+  await load(browser, filename);
+  ok(!browser.contentDocument.loadedFromPrototype, `${filename} should not use prototype.`);
+  const contentWithoutPrototype = browser.contentDocument.documentElement.outerHTML;
+
+  // Load the page with the prototype document cache enabled. The prototype should
+  // be built from the source file.
+  await SpecialPowers.pushPrefEnv({ set: [["dom.prototype_document_cache.enabled", true]] });
+  await load(browser, filename);
+  ok(browser.contentDocument.loadedFromPrototype, `${filename} should load from prototype.`);
+  const contentWithPrototype = browser.contentDocument.documentElement.outerHTML;
+  is(contentWithPrototype, contentWithoutPrototype, `${filename} document contents should be the same.`);
+}
+
+add_task(async function test_prototype_document() {
+  await compare("no_whitespace.xhtml");
+  await compare("whitespace.xhtml");
+  // TODO: Test whitespace within XUL elements, since it is handled differently
+  // with and without the prototype sink (bug 1544567).
+});
+
+add_task(async function test_prototype_document_form() {
+  let browser = document.getElementById("browser");
+  await load(browser, "form.xhtml");
+  ok(browser.contentDocument.loadedFromPrototype, `form.xhtml should load from prototype.`);
+  browser.contentDocument.getElementById("input").value = "test";
+  await load(browser, "form.xhtml");
+  is(browser.contentDocument.getElementById("input").value, "test", "input value should persist after reload");
+});
+]]></script>
+</body>
+</window>
diff --git a/dom/prototype/tests/chrome/whitespace.xhtml b/dom/prototype/tests/chrome/whitespace.xhtml
new file mode 100644
index 0000000000..0f121973c7
--- /dev/null
+++ b/dom/prototype/tests/chrome/whitespace.xhtml
@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/strict.dtd">
+<html xmlns="http://www.w3.org/TR/xhtml1/strict">
+  <body>
+    Hello
+    <p>there!</p>
+  </body>
+</html>