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diff --git a/layout/style/CSSVariableDeclarations.h b/layout/style/CSSVariableDeclarations.h
new file mode 100644
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+++ b/layout/style/CSSVariableDeclarations.h
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+/* vim: set shiftwidth=2 tabstop=8 autoindent cindent expandtab: */
+/* 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/. */
+
+/* CSS Custom Property assignments for a Declaration at a given priority */
+
+#ifndef mozilla_CSSVariableDeclarations_h
+#define mozilla_CSSVariableDeclarations_h
+
+#include "nsDataHashtable.h"
+
+namespace mozilla {
+class CSSVariableResolver;
+} // namespace mozilla
+struct nsRuleData;
+
+namespace mozilla {
+
+class CSSVariableDeclarations
+{
+public:
+  CSSVariableDeclarations();
+  CSSVariableDeclarations(const CSSVariableDeclarations& aOther);
+#ifdef DEBUG
+  ~CSSVariableDeclarations();
+#endif
+  CSSVariableDeclarations& operator=(const CSSVariableDeclarations& aOther);
+
+  /**
+   * Returns whether this set of variable declarations includes a variable
+   * with a given name.
+   *
+   * @param aName The variable name (not including any "--" prefix that would
+   *   be part of the custom property name).
+   */
+  bool Has(const nsAString& aName) const;
+
+  /**
+   * Represents the type of a variable value.
+   */
+  enum Type {
+    eTokenStream,  // a stream of CSS tokens (the usual type for variables)
+    eInitial,      // 'initial'
+    eInherit,      // 'inherit'
+    eUnset         // 'unset'
+  };
+
+  /**
+   * Gets the value of a variable in this set of variable declarations.
+   *
+   * @param aName The variable name (not including any "--" prefix that would
+   *   be part of the custom property name).
+   * @param aType Out parameter into which the type of the variable value will
+   *   be stored.
+   * @param aValue Out parameter into which the value of the variable will
+   *   be stored.  If the variable is 'initial', 'inherit' or 'unset', this will
+   *   be the empty string.
+   * @return Whether a variable with the given name was found.  When false
+   *   is returned, aType and aValue will not be modified.
+   */
+  bool Get(const nsAString& aName, Type& aType, nsString& aValue) const;
+
+  /**
+   * Adds or modifies an existing entry in this set of variable declarations
+   * to have the value 'initial'.
+   *
+   * @param aName The variable name (not including any "--" prefix that would
+   *   be part of the custom property name) whose value is to be set.
+   */
+  void PutInitial(const nsAString& aName);
+
+  /**
+   * Adds or modifies an existing entry in this set of variable declarations
+   * to have the value 'inherit'.
+   *
+   * @param aName The variable name (not including any "--" prefix that would
+   *   be part of the custom property name) whose value is to be set.
+   */
+  void PutInherit(const nsAString& aName);
+
+  /**
+   * Adds or modifies an existing entry in this set of variable declarations
+   * to have the value 'unset'.
+   *
+   * @param aName The variable name (not including any "--" prefix that would
+   *   be part of the custom property name) whose value is to be set.
+   */
+  void PutUnset(const nsAString& aName);
+
+  /**
+   * Adds or modifies an existing entry in this set of variable declarations
+   * to have a token stream value.
+   *
+   * @param aName The variable name (not including any "--" prefix that would
+   *   be part of the custom property name) whose value is to be set.
+   * @param aTokenStream The CSS token stream.
+   */
+  void PutTokenStream(const nsAString& aName, const nsString& aTokenStream);
+
+  /**
+   * Removes an entry in this set of variable declarations.
+   *
+   * @param aName The variable name (not including any "--" prefix that would
+   *   be part of the custom property name) whose entry is to be removed.
+   */
+  void Remove(const nsAString& aName);
+
+  /**
+   * Returns the number of entries in this set of variable declarations.
+   */
+  uint32_t Count() const { return mVariables.Count(); }
+
+  /**
+   * Copies each variable value from this object into aRuleData, unless that
+   * variable already exists on aRuleData.
+   */
+  void MapRuleInfoInto(nsRuleData* aRuleData);
+
+  /**
+   * Copies the variables from this object into aResolver, marking them as
+   * specified values.
+   */
+  void AddVariablesToResolver(CSSVariableResolver* aResolver) const;
+
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+
+private:
+  /**
+   * Adds all the variable declarations from aOther into this object.
+   */
+  void CopyVariablesFrom(const CSSVariableDeclarations& aOther);
+
+  nsDataHashtable<nsStringHashKey, nsString> mVariables;
+};
+
+} // namespace mozilla
+
+#endif