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diff --git a/layout/style/MappedDeclarationsBuilder.h b/layout/style/MappedDeclarationsBuilder.h
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+++ b/layout/style/MappedDeclarationsBuilder.h
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+/* -*- 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/. */
+
+/* Representation of a declaration block used for attribute mapping */
+
+#ifndef mozilla_MappedDeclarationsBuilder_h
+#define mozilla_MappedDeclarationsBuilder_h
+
+#include "mozilla/FontPropertyTypes.h"
+#include "mozilla/ServoBindingTypes.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/ServoBindings.h"
+#include "nsCSSPropertyID.h"
+#include "nsCSSValue.h"
+#include "nsColor.h"
+
+class nsAttrValue;
+
+namespace mozilla {
+
+// This provides a convenient interface for attribute mappers
+// (MapAttributesIntoRule) to modify the presentation attribute declaration
+// block for a given element.
+class MOZ_STACK_CLASS MappedDeclarationsBuilder final {
+ public:
+  explicit MappedDeclarationsBuilder(
+      dom::Element& aElement, dom::Document& aDoc,
+      StyleLockedDeclarationBlock* aDecls = nullptr)
+      : mDocument(aDoc), mElement(aElement), mDecls(aDecls) {
+    if (mDecls) {
+      Servo_DeclarationBlock_Clear(mDecls);
+    }
+  }
+
+  ~MappedDeclarationsBuilder() {
+    MOZ_ASSERT(!mDecls, "Forgot to take the block?");
+  }
+
+  dom::Document& Document() { return mDocument; }
+
+  already_AddRefed<StyleLockedDeclarationBlock> TakeDeclarationBlock() {
+    return mDecls.forget();
+  }
+
+  // Check if we already contain a certain longhand
+  bool PropertyIsSet(nsCSSPropertyID aId) const {
+    return mDecls && Servo_DeclarationBlock_PropertyIsSet(mDecls, aId);
+  }
+
+  // Set a property to an identifier (string)
+  void SetIdentStringValue(nsCSSPropertyID aId, const nsString& aValue) {
+    RefPtr<nsAtom> atom = NS_AtomizeMainThread(aValue);
+    SetIdentAtomValue(aId, atom);
+  }
+
+  void SetIdentStringValueIfUnset(nsCSSPropertyID aId, const nsString& aValue) {
+    if (!PropertyIsSet(aId)) {
+      SetIdentStringValue(aId, aValue);
+    }
+  }
+
+  void SetIdentAtomValue(nsCSSPropertyID aId, nsAtom* aValue);
+
+  void SetIdentAtomValueIfUnset(nsCSSPropertyID aId, nsAtom* aValue) {
+    if (!PropertyIsSet(aId)) {
+      SetIdentAtomValue(aId, aValue);
+    }
+  }
+
+  // Set a property to a keyword (usually NS_STYLE_* or StyleFoo::*)
+  void SetKeywordValue(nsCSSPropertyID aId, int32_t aValue) {
+    Servo_DeclarationBlock_SetKeywordValue(&EnsureDecls(), aId, aValue);
+  }
+
+  void SetKeywordValueIfUnset(nsCSSPropertyID aId, int32_t aValue) {
+    if (!PropertyIsSet(aId)) {
+      SetKeywordValue(aId, aValue);
+    }
+  }
+
+  template <typename T,
+            typename = typename std::enable_if<std::is_enum<T>::value>::type>
+  void SetKeywordValue(nsCSSPropertyID aId, T aValue) {
+    static_assert(EnumTypeFitsWithin<T, int32_t>::value,
+                  "aValue must be an enum that fits within 32 bits");
+    SetKeywordValue(aId, static_cast<int32_t>(aValue));
+  }
+  template <typename T,
+            typename = typename std::enable_if<std::is_enum<T>::value>::type>
+  void SetKeywordValueIfUnset(nsCSSPropertyID aId, T aValue) {
+    static_assert(EnumTypeFitsWithin<T, int32_t>::value,
+                  "aValue must be an enum that fits within 32 bits");
+    SetKeywordValueIfUnset(aId, static_cast<int32_t>(aValue));
+  }
+
+  // Set a property to an integer value
+  void SetIntValue(nsCSSPropertyID aId, int32_t aValue) {
+    Servo_DeclarationBlock_SetIntValue(&EnsureDecls(), aId, aValue);
+  }
+
+  // Set "math-depth: <integer>" or "math-depth: add(<integer>)"
+  void SetMathDepthValue(int32_t aValue, bool aIsRelative) {
+    Servo_DeclarationBlock_SetMathDepthValue(&EnsureDecls(), aValue,
+                                             aIsRelative);
+  }
+
+  // Set "counter-reset: list-item <integer>".  If aIsReversed is true then
+  // "list-item" instead becomes "reversed(list-item)".
+  void SetCounterResetListItem(int32_t aValue, bool aIsReversed) {
+    Servo_DeclarationBlock_SetCounterResetListItem(&EnsureDecls(), aValue,
+                                                   aIsReversed);
+  }
+
+  // Set "counter-set: list-item <integer>".
+  void SetCounterSetListItem(int32_t aValue) {
+    Servo_DeclarationBlock_SetCounterSetListItem(&EnsureDecls(), aValue);
+  }
+
+  // Set a property to a pixel value
+  void SetPixelValue(nsCSSPropertyID aId, float aValue) {
+    Servo_DeclarationBlock_SetPixelValue(&EnsureDecls(), aId, aValue);
+  }
+
+  void SetPixelValueIfUnset(nsCSSPropertyID aId, float aValue) {
+    if (!PropertyIsSet(aId)) {
+      SetPixelValue(aId, aValue);
+    }
+  }
+
+  void SetLengthValue(nsCSSPropertyID aId, const nsCSSValue& aValue) {
+    MOZ_ASSERT(aValue.IsLengthUnit());
+    Servo_DeclarationBlock_SetLengthValue(
+        &EnsureDecls(), aId, aValue.GetFloatValue(), aValue.GetUnit());
+  }
+
+  // Set a property to a percent value
+  void SetPercentValue(nsCSSPropertyID aId, float aValue) {
+    Servo_DeclarationBlock_SetPercentValue(&EnsureDecls(), aId, aValue);
+  }
+
+  void SetPercentValueIfUnset(nsCSSPropertyID aId, float aValue) {
+    if (!PropertyIsSet(aId)) {
+      SetPercentValue(aId, aValue);
+    }
+  }
+
+  // Set a property to `auto`
+  void SetAutoValue(nsCSSPropertyID aId) {
+    Servo_DeclarationBlock_SetAutoValue(&EnsureDecls(), aId);
+  }
+
+  void SetAutoValueIfUnset(nsCSSPropertyID aId) {
+    if (!PropertyIsSet(aId)) {
+      SetAutoValue(aId);
+    }
+  }
+
+  // Set a property to `currentcolor`
+  void SetCurrentColor(nsCSSPropertyID aId) {
+    Servo_DeclarationBlock_SetCurrentColor(&EnsureDecls(), aId);
+  }
+
+  void SetCurrentColorIfUnset(nsCSSPropertyID aId) {
+    if (!PropertyIsSet(aId)) {
+      SetCurrentColor(aId);
+    }
+  }
+
+  // Set a property to an RGBA nscolor value
+  void SetColorValue(nsCSSPropertyID aId, nscolor aValue) {
+    Servo_DeclarationBlock_SetColorValue(&EnsureDecls(), aId, aValue);
+  }
+
+  void SetColorValueIfUnset(nsCSSPropertyID aId, nscolor aValue) {
+    if (!PropertyIsSet(aId)) {
+      SetColorValue(aId, aValue);
+    }
+  }
+
+  // Set font-family to a string
+  void SetFontFamily(const nsACString& aValue) {
+    Servo_DeclarationBlock_SetFontFamily(&EnsureDecls(), &aValue);
+  }
+
+  // Add a quirks-mode override to the decoration color of elements nested in
+  // <a>
+  void SetTextDecorationColorOverride() {
+    Servo_DeclarationBlock_SetTextDecorationColorOverride(&EnsureDecls());
+  }
+
+  void SetBackgroundImage(const nsAttrValue& value);
+
+  void SetAspectRatio(float aWidth, float aHeight) {
+    Servo_DeclarationBlock_SetAspectRatio(&EnsureDecls(), aWidth, aHeight);
+  }
+
+  const nsAttrValue* GetAttr(nsAtom* aName) {
+    MOZ_ASSERT(mElement.IsAttributeMapped(aName));
+    return mElement.GetParsedAttr(aName);
+  }
+
+ private:
+  StyleLockedDeclarationBlock& EnsureDecls() {
+    if (!mDecls) {
+      mDecls = Servo_DeclarationBlock_CreateEmpty().Consume();
+    }
+    return *mDecls;
+  }
+
+  dom::Document& mDocument;
+  dom::Element& mElement;
+  RefPtr<StyleLockedDeclarationBlock> mDecls;
+};
+
+}  // namespace mozilla
+
+#endif