Implement unset value handling

Source/core/css/parser/CSSPropertyParser.cpp

Index: Source/core/css/parser/CSSPropertyParser.cpp
diff --git a/Source/core/css/parser/CSSPropertyParser.cpp b/Source/core/css/parser/CSSPropertyParser.cpp
index e0b740f6e03ce13eb03a386b1d1b7434656ed079..fd64d241ac4e05c296a83059710bb52da4c8305d 100644
--- a/Source/core/css/parser/CSSPropertyParser.cpp
+++ b/Source/core/css/parser/CSSPropertyParser.cpp
@@ -469,12 +469,16 @@ bool CSSPropertyParser::parseValue(CSSPropertyID propId, bool important)
             return false;
         addExpandedPropertyForValue(propId, cssValuePool().createInheritedValue(), important);
         return true;
-    }
-    else if (id == CSSValueInitial) {
+    } else if (id == CSSValueInitial) {
         if (num != 1)
             return false;
         addExpandedPropertyForValue(propId, cssValuePool().createExplicitInitialValue(), important);
         return true;
+    } else if (id == CSSValueUnset) {
+        if (num != 1)
+            return false;
+        addExpandedPropertyForValue(propId, cssValuePool().createUnsetValue(), important);
+        return true;
     }
 
     if (CSSParserFastPaths::isKeywordPropertyID(propId)) {