--- a/layout/style/moz.build
+++ b/layout/style/moz.build
@@ -59,16 +59,17 @@ EXPORTS += [
     'nsIStyleRuleProcessor.h',
     'nsLayoutStylesheetCache.h',
     'nsMediaFeatures.h',
     'nsMediaList.h',
     'nsRuleData.h',
     'nsRuleNode.h',
     'nsRuleProcessorData.h',
     'nsRuleWalker.h',
+    'nsStyleAutoArray.h',
     'nsStyleConsts.h',
     'nsStyleContext.h',
     'nsStyleCoord.h',
     'nsStyleSet.h',
     'nsStyleStruct.h',
     'nsStyleStructFwd.h',
     'nsStyleStructInlines.h',
     'nsStyleTransformMatrix.h',

new file mode 100644
--- /dev/null
+++ b/layout/style/nsStyleAutoArray.h
@@ -0,0 +1,78 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 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/. */
+
+#ifndef nsStyleAutoArray_h_
+#define nsStyleAutoArray_h_
+
+#include "nsTArray.h"
+#include "mozilla/Assertions.h"
+
+/**
+ * An array of objects, similar to AutoTArray<T,1> but which is memmovable. It
+ * always has length >= 1.
+ */
+template<typename T>
+class nsStyleAutoArray
+{
+public:
+  // This constructor places a single element in mFirstElement.
+  enum WithSingleInitialElement { WITH_SINGLE_INITIAL_ELEMENT };
+  explicit nsStyleAutoArray(WithSingleInitialElement) {}
+  nsStyleAutoArray(const nsStyleAutoArray& aOther) { *this = aOther; }
+  nsStyleAutoArray& operator=(const nsStyleAutoArray& aOther) {
+    mFirstElement = aOther.mFirstElement;
+    mOtherElements = aOther.mOtherElements;
+    return *this;
+  }
+
+  bool operator==(const nsStyleAutoArray& aOther) const {
+    return Length() == aOther.Length() &&
+           mFirstElement == aOther.mFirstElement &&
+           mOtherElements == aOther.mOtherElements;
+  }
+  bool operator!=(const nsStyleAutoArray& aOther) const {
+    return !(*this == aOther);
+  }
+
+  nsStyleAutoArray& operator=(nsStyleAutoArray&& aOther) {
+    mFirstElement = aOther.mFirstElement;
+    mOtherElements.SwapElements(aOther.mOtherElements);
+
+    return *this;
+  }
+
+  size_t Length() const {
+    return mOtherElements.Length() + 1;
+  }
+  const T& operator[](size_t aIndex) const {
+    return aIndex == 0 ? mFirstElement : mOtherElements[aIndex - 1];
+  }
+  T& operator[](size_t aIndex) {
+    return aIndex == 0 ? mFirstElement : mOtherElements[aIndex - 1];
+  }
+
+  void EnsureLengthAtLeast(size_t aMinLen) {
+    if (aMinLen > 0) {
+      mOtherElements.EnsureLengthAtLeast(aMinLen - 1);
+    }
+  }
+
+  void SetLengthNonZero(size_t aNewLen) {
+    MOZ_ASSERT(aNewLen > 0);
+    mOtherElements.SetLength(aNewLen - 1);
+  }
+
+  void TruncateLengthNonZero(size_t aNewLen) {
+    MOZ_ASSERT(aNewLen > 0);
+    MOZ_ASSERT(aNewLen <= Length());
+    mOtherElements.TruncateLength(aNewLen - 1);
+  }
+
+private:
+  T mFirstElement;
+  nsTArray<T> mOtherElements;
+};
+
+#endif /* nsStyleAutoArray_h_ */

--- a/layout/style/nsStyleStruct.h
+++ b/layout/style/nsStyleStruct.h
@@ -19,16 +19,17 @@
 #include "mozilla/SheetType.h"
 #include "mozilla/StaticPtr.h"
 #include "mozilla/StyleComplexColor.h"
 #include "mozilla/UniquePtr.h"
 #include "nsColor.h"
 #include "nsCoord.h"
 #include "nsMargin.h"
 #include "nsFont.h"
+#include "nsStyleAutoArray.h"
 #include "nsStyleCoord.h"
 #include "nsStyleConsts.h"
 #include "nsChangeHint.h"
 #include "nsPresContext.h"
 #include "nsCOMPtr.h"
 #include "nsCOMArray.h"
 #include "nsTArray.h"
 #include "nsCSSValue.h"
@@ -597,82 +598,16 @@ struct MOZ_NEEDS_MEMMOVABLE_MEMBERS nsSt
       FreeByObjectID(mozilla::eArenaObjectID_nsStyleColor, this);
   }
 
   // Don't add ANY members to this struct!  We can achieve caching in the rule
   // tree (rather than the style tree) by letting color stay by itself! -dwh
   nscolor mColor;                 // [inherited]
 };
 
-/**
- * An array of objects, similar to AutoTArray<T,1> but which is memmovable. It
- * always has length >= 1.
- */
-template<typename T>
-class nsStyleAutoArray
-{
-public:
-  // This constructor places a single element in mFirstElement.
-  enum WithSingleInitialElement { WITH_SINGLE_INITIAL_ELEMENT };
-  explicit nsStyleAutoArray(WithSingleInitialElement) {}
-  nsStyleAutoArray(const nsStyleAutoArray& aOther) { *this = aOther; }
-  nsStyleAutoArray& operator=(const nsStyleAutoArray& aOther) {
-    mFirstElement = aOther.mFirstElement;
-    mOtherElements = aOther.mOtherElements;
-    return *this;
-  }
-
-  bool operator==(const nsStyleAutoArray& aOther) const {
-    return Length() == aOther.Length() &&
-           mFirstElement == aOther.mFirstElement &&
-           mOtherElements == aOther.mOtherElements;
-  }
-  bool operator!=(const nsStyleAutoArray& aOther) const {
-    return !(*this == aOther);
-  }
-
-  nsStyleAutoArray& operator=(nsStyleAutoArray&& aOther) {
-    mFirstElement = aOther.mFirstElement;
-    mOtherElements.SwapElements(aOther.mOtherElements);
-
-    return *this;
-  }
-
-  size_t Length() const {
-    return mOtherElements.Length() + 1;
-  }
-  const T& operator[](size_t aIndex) const {
-    return aIndex == 0 ? mFirstElement : mOtherElements[aIndex - 1];
-  }
-  T& operator[](size_t aIndex) {
-    return aIndex == 0 ? mFirstElement : mOtherElements[aIndex - 1];
-  }
-
-  void EnsureLengthAtLeast(size_t aMinLen) {
-    if (aMinLen > 0) {
-      mOtherElements.EnsureLengthAtLeast(aMinLen - 1);
-    }
-  }
-
-  void SetLengthNonZero(size_t aNewLen) {
-    MOZ_ASSERT(aNewLen > 0);
-    mOtherElements.SetLength(aNewLen - 1);
-  }
-
-  void TruncateLengthNonZero(size_t aNewLen) {
-    MOZ_ASSERT(aNewLen > 0);
-    MOZ_ASSERT(aNewLen <= Length());
-    mOtherElements.TruncateLength(aNewLen - 1);
-  }
-
-private:
-  T mFirstElement;
-  nsTArray<T> mOtherElements;
-};
-
 struct nsStyleImageLayers {
   // Indices into kBackgroundLayerTable and kMaskLayerTable
   enum {
     shorthand = 0,
     color,
     image,
     repeat,
     positionX,