--- a/layout/style/nsCSSValue.cpp
+++ b/layout/style/nsCSSValue.cpp
@@ -221,16 +221,20 @@ nsCSSValue::nsCSSValue(const nsCSSValue&
   else if (eCSSUnit_GridTemplateAreas == mUnit) {
     mValue.mGridTemplateAreas = aCopy.mValue.mGridTemplateAreas;
     mValue.mGridTemplateAreas->AddRef();
   }
   else if (eCSSUnit_FontFamilyList == mUnit) {
     mValue.mFontFamilyList = aCopy.mValue.mFontFamilyList;
     mValue.mFontFamilyList->AddRef();
   }
+  else if (eCSSUnit_AtomIdent == mUnit) {
+    mValue.mAtom = aCopy.mValue.mAtom;
+    mValue.mAtom->AddRef();
+  }
   else {
     MOZ_ASSERT(false, "unknown unit");
   }
 }
 
 nsCSSValue& nsCSSValue::operator=(const nsCSSValue& aCopy)
 {
   if (this != &aCopy) {
@@ -316,16 +320,19 @@ bool nsCSSValue::operator==(const nsCSSV
                                        aOther.mValue.mPairList);
     }
     else if (eCSSUnit_GridTemplateAreas == mUnit) {
       return *mValue.mGridTemplateAreas == *aOther.mValue.mGridTemplateAreas;
     }
     else if (eCSSUnit_FontFamilyList == mUnit) {
       return *mValue.mFontFamilyList == *aOther.mValue.mFontFamilyList;
     }
+    else if (eCSSUnit_AtomIdent == mUnit) {
+      return mValue.mAtom == aOther.mValue.mAtom;
+    }
     else {
       return mValue.mFloat == aOther.mValue.mFloat;
     }
   }
   return false;
 }
 
 double
@@ -448,16 +455,18 @@ void nsCSSValue::DoReset()
   } else if (eCSSUnit_SharedList == mUnit) {
     DO_RELEASE(mSharedList);
   } else if (eCSSUnit_PairList == mUnit) {
     DO_RELEASE(mPairList);
   } else if (eCSSUnit_GridTemplateAreas == mUnit) {
     DO_RELEASE(mGridTemplateAreas);
   } else if (eCSSUnit_FontFamilyList == mUnit) {
     DO_RELEASE(mFontFamilyList);
+  } else if (eCSSUnit_AtomIdent == mUnit) {
+    DO_RELEASE(mAtom);
   }
   mUnit = eCSSUnit_Null;
 }
 
 #undef DO_RELEASE
 
 void nsCSSValue::SetIntValue(int32_t aValue, nsCSSUnit aUnit)
 {
@@ -970,16 +979,26 @@ nsCSSValue::BufferFromString(const nsStr
 
   char16_t* data = static_cast<char16_t*>(buffer->Data());
   nsCharTraits<char16_t>::copy(data, aValue.get(), length);
   // Null-terminate.
   data[length] = 0;
   return buffer.forget();
 }
 
+void
+nsCSSValue::AtomizeIdentValue()
+{
+  MOZ_ASSERT(mUnit == eCSSUnit_Ident);
+  nsCOMPtr<nsIAtom> atom = NS_Atomize(GetStringBufferValue());
+  Reset();
+  mUnit = eCSSUnit_AtomIdent;
+  mValue.mAtom = atom.forget().take();
+}
+
 namespace {
 
 struct CSSValueSerializeCalcOps {
   CSSValueSerializeCalcOps(nsCSSPropertyID aProperty, nsAString& aResult,
                            nsCSSValue::Serialization aSerialization)
     : mProperty(aProperty),
       mResult(aResult),
       mValueSerialization(aSerialization)
@@ -1961,16 +1980,19 @@ nsCSSValue::AppendToString(nsCSSProperty
     nsStyleUtil::AppendEscapedCSSString(areas->mTemplates[0], aResult);
     for (uint32_t i = 1; i < areas->mTemplates.Length(); i++) {
       aResult.Append(char16_t(' '));
       nsStyleUtil::AppendEscapedCSSString(areas->mTemplates[i], aResult);
     }
   } else if (eCSSUnit_FontFamilyList == unit) {
     nsStyleUtil::AppendEscapedCSSFontFamilyList(*mValue.mFontFamilyList,
                                                 aResult);
+  } else if (eCSSUnit_AtomIdent == unit) {
+    nsDependentAtomString buffer(GetAtomValue());
+    nsStyleUtil::AppendEscapedCSSIdent(buffer, aResult);
   }
 
   switch (unit) {
     case eCSSUnit_Null:         break;
     case eCSSUnit_Auto:         aResult.AppendLiteral("auto");     break;
     case eCSSUnit_Inherit:      aResult.AppendLiteral("inherit");  break;
     case eCSSUnit_Initial:      aResult.AppendLiteral("initial");  break;
     case eCSSUnit_Unset:        aResult.AppendLiteral("unset");    break;
@@ -1981,16 +2003,17 @@ nsCSSValue::AppendToString(nsCSSProperty
     case eCSSUnit_Dummy:
     case eCSSUnit_DummyInherit:
       MOZ_ASSERT(false, "should never serialize");
       break;
 
     case eCSSUnit_FontFamilyList: break;
     case eCSSUnit_String:       break;
     case eCSSUnit_Ident:        break;
+    case eCSSUnit_AtomIdent:    break;
     case eCSSUnit_URL:          break;
     case eCSSUnit_Image:        break;
     case eCSSUnit_Element:      break;
     case eCSSUnit_Array:        break;
     case eCSSUnit_Attr:
     case eCSSUnit_Cubic_Bezier:
     case eCSSUnit_Steps:
     case eCSSUnit_Symbols:
@@ -2178,16 +2201,20 @@ nsCSSValue::SizeOfExcludingThis(mozilla:
     case eCSSUnit_GridTemplateAreas:
       n += mValue.mGridTemplateAreas->SizeOfIncludingThis(aMallocSizeOf);
       break;
 
     case eCSSUnit_FontFamilyList:
       n += mValue.mFontFamilyList->SizeOfIncludingThis(aMallocSizeOf);
       break;
 
+    // Atom is always shared, and thus should not be counted.
+    case eCSSUnit_AtomIdent:
+      break;
+
     // Int: nothing extra to measure.
     case eCSSUnit_Integer:
     case eCSSUnit_Enumerated:
     case eCSSUnit_EnumColor:
       break;
 
     // Integer Color: nothing extra to measure.
     case eCSSUnit_RGBColor:

--- a/layout/style/nsCSSValue.h
+++ b/layout/style/nsCSSValue.h
@@ -29,16 +29,17 @@
 #include "nsString.h"
 #include "nsStringBuffer.h"
 #include "nsTArray.h"
 #include "nsStyleConsts.h"
 #include "nsStyleCoord.h"
 #include "gfxFontFamilyList.h"
 
 class imgRequestProxy;
+class nsIAtom;
 class nsIContent;
 class nsIDocument;
 class nsIPrincipal;
 class nsIURI;
 class nsPresContext;
 template <class T>
 class nsPtrHashKey;
 
@@ -484,16 +485,19 @@ enum nsCSSUnit {
   eCSSUnit_SharedList   = 55,     // (nsCSSValueSharedList*) same as list
                                   //   but reference counted and shared
   eCSSUnit_PairList     = 56,     // (nsCSSValuePairList*) list of value pairs
   eCSSUnit_PairListDep  = 57,     // (nsCSSValuePairList*) same as PairList
                                   //   but does not own the list
 
   eCSSUnit_FontFamilyList = 58,   // (FontFamilyList*) value
 
+  // Atom units
+  eCSSUnit_AtomIdent    = 60,     // (nsIAtom*) for its string as an identifier
+
   eCSSUnit_Integer      = 70,     // (int) simple value
   eCSSUnit_Enumerated   = 71,     // (int) value has enumerated meaning
 
   eCSSUnit_EnumColor           = 80,   // (int) enumerated color (kColorKTable)
   eCSSUnit_RGBColor            = 81,   // (nscolor) an opaque RGBA value specified as rgb()
   eCSSUnit_RGBAColor           = 82,   // (nscolor) an RGBA value specified as rgba()
   eCSSUnit_HexColor            = 83,   // (nscolor) an opaque RGBA value specified as #rrggbb
   eCSSUnit_ShortHexColor       = 84,   // (nscolor) an opaque RGBA value specified as #rgb
@@ -873,16 +877,21 @@ public:
   nscoord GetPixelLength() const;
 
   nsCSSValueFloatColor* GetFloatColorValue() const
   {
     MOZ_ASSERT(IsFloatColorUnit(), "not a float color value");
     return mValue.mFloatColor;
   }
 
+  nsIAtom* GetAtomValue() const {
+    MOZ_ASSERT(mUnit == eCSSUnit_AtomIdent);
+    return mValue.mAtom;
+  }
+
   void Reset()  // sets to null
   {
     if (mUnit != eCSSUnit_Null)
       DoReset();
   }
 private:
   void DoReset();
 
@@ -957,16 +966,19 @@ public:
   // Checks if this is a function value with the specified function id.
   bool EqualsFunction(nsCSSKeyword aFunctionId) const;
 
   // Returns an already addrefed buffer.  Guaranteed to return non-null.
   // (Will abort on allocation failure.)
   static already_AddRefed<nsStringBuffer>
     BufferFromString(const nsString& aValue);
 
+  // Convert the given Ident value into AtomIdent.
+  void AtomizeIdentValue();
+
   size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
 
   static void
   AppendSidesShorthandToString(const nsCSSPropertyID aProperties[],
                                const nsCSSValue* aValues[],
                                nsAString& aString,
                                Serialization aSerialization);
   static void
@@ -1001,16 +1013,17 @@ protected:
   nsCSSUnit mUnit;
   union {
     int32_t    mInt;
     float      mFloat;
     // Note: the capacity of the buffer may exceed the length of the string.
     // If we're of a string type, mString is not null.
     nsStringBuffer* MOZ_OWNING_REF mString;
     nscolor    mColor;
+    nsIAtom* MOZ_OWNING_REF mAtom;
     Array* MOZ_OWNING_REF mArray;
     mozilla::css::URLValue* MOZ_OWNING_REF mURL;
     mozilla::css::ImageValue* MOZ_OWNING_REF mImage;
     mozilla::css::GridTemplateAreasValue* MOZ_OWNING_REF mGridTemplateAreas;
     nsCSSValueGradient* MOZ_OWNING_REF mGradient;
     nsCSSValueTokenStream* MOZ_OWNING_REF mTokenStream;
     nsCSSValuePair_heap* MOZ_OWNING_REF mPair;
     nsCSSRect_heap* MOZ_OWNING_REF mRect;