diff --git a/mozilla/content/html/style/src/nsICSSStyleRule.h b/mozilla/content/html/style/src/nsICSSStyleRule.h
index 77104d6d703..2bd21cd995f 100644
--- a/mozilla/content/html/style/src/nsICSSStyleRule.h
+++ b/mozilla/content/html/style/src/nsICSSStyleRule.h
@@ -171,7 +171,7 @@ public:
   virtual nsIStyleRule* GetImportantRule(void) = 0;
 
   // Hook for inspector.
-  virtual nsresult GetValue(nsCSSProperty aProperty, nsCSSValue& aValue);
+  virtual nsresult GetValue(nsCSSProperty aProperty, nsCSSValue& aValue) = 0;
 };
 
 extern NS_EXPORT nsresult
diff --git a/mozilla/layout/style/nsICSSStyleRule.h b/mozilla/layout/style/nsICSSStyleRule.h
index 77104d6d703..2bd21cd995f 100644
--- a/mozilla/layout/style/nsICSSStyleRule.h
+++ b/mozilla/layout/style/nsICSSStyleRule.h
@@ -171,7 +171,7 @@ public:
   virtual nsIStyleRule* GetImportantRule(void) = 0;
 
   // Hook for inspector.
-  virtual nsresult GetValue(nsCSSProperty aProperty, nsCSSValue& aValue);
+  virtual nsresult GetValue(nsCSSProperty aProperty, nsCSSValue& aValue) = 0;
 };
 
 extern NS_EXPORT nsresult