new file mode 100644
--- /dev/null
+++ b/layout/style/DocumentStyleRootIterator.cpp
@@ -0,0 +1,41 @@
+/* -*- 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 http://mozilla.org/MPL/2.0/. */
+
+#include "DocumentStyleRootIterator.h"
+
+#include "nsContentUtils.h"
+
+namespace mozilla {
+
+DocumentStyleRootIterator::DocumentStyleRootIterator(nsIDocument* aDocument)
+  : mPosition(0)
+{
+  MOZ_COUNT_CTOR(DocumentStyleRootIterator);
+  if (Element* root = aDocument->GetRootElement()) {
+    mStyleRoots.AppendElement(root);
+  }
+  nsContentUtils::AppendDocumentLevelNativeAnonymousContentTo(
+      aDocument, mStyleRoots);
+}
+
+Element*
+DocumentStyleRootIterator::GetNextStyleRoot()
+{
+  for (;;) {
+    if (mPosition >= mStyleRoots.Length()) {
+      return nullptr;
+    }
+
+    nsIContent* next = mStyleRoots[mPosition];
+    ++mPosition;
+
+    if (next->IsElement()) {
+      return next->AsElement();
+    }
+  }
+}
+
+} // namespace mozilla

new file mode 100644
--- /dev/null
+++ b/layout/style/DocumentStyleRootIterator.h
@@ -0,0 +1,37 @@
+/* -*- 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 http://mozilla.org/MPL/2.0/. */
+
+#ifndef DocumentStyleRootIterator_h
+#define DocumentStyleRootIterator_h
+
+#include "nsTArray.h"
+
+class nsIContent;
+class nsIDocument;
+
+namespace mozilla {
+
+/**
+ * DocumentStyleRootIterator traverses the roots of the document from the
+ * perspective of the Servo-backed style system.  This will first traverse
+ * the document root, followed by any document level native anonymous content.
+ */
+class DocumentStyleRootIterator
+{
+public:
+  explicit DocumentStyleRootIterator(nsIDocument* aDocument);
+  ~DocumentStyleRootIterator() { MOZ_COUNT_DTOR(DocumentStyleRootIterator); }
+
+  Element* GetNextStyleRoot();
+
+private:
+  AutoTArray<nsIContent*, 8> mStyleRoots;
+  uint32_t mPosition;
+};
+
+} // namespace mozilla
+
+#endif // DocumentStyleRootIterator_h

--- a/layout/style/moz.build
+++ b/layout/style/moz.build
@@ -83,16 +83,17 @@ EXPORTS.mozilla += [
     'AnimationCollection.h',
     'CSSEnabledState.h',
     'CSSStyleSheet.h',
     'CSSVariableDeclarations.h',
     'CSSVariableResolver.h',
     'CSSVariableValues.h',
     'DeclarationBlock.h',
     'DeclarationBlockInlines.h',
+    'DocumentStyleRootIterator.h',
     'HandleRefPtr.h',
     'IncrementalClearCOMRuleArray.h',
     'LayerAnimationInfo.h',
     'RuleNodeCacheConditions.h',
     'RuleProcessorCache.h',
     'ServoBindingList.h',
     'ServoBindings.h',
     'ServoBindingTypes.h',
@@ -148,16 +149,17 @@ UNIFIED_SOURCES += [
     'CSS.cpp',
     'CSSLexer.cpp',
     'CSSRuleList.cpp',
     'CSSStyleSheet.cpp',
     'CSSVariableDeclarations.cpp',
     'CSSVariableResolver.cpp',
     'CSSVariableValues.cpp',
     'Declaration.cpp',
+    'DocumentStyleRootIterator.cpp',
     'ErrorReporter.cpp',
     'FontFace.cpp',
     'FontFaceSet.cpp',
     'FontFaceSetIterator.cpp',
     'ImageLoader.cpp',
     'IncrementalClearCOMRuleArray.cpp',
     'LayerAnimationInfo.cpp',
     'Loader.cpp',