diff --git a/layout/style/nsRuleWalker.h b/layout/style/nsRuleWalker.h
index 1aebed7754d..9faa09e5446 100644
--- a/layout/style/nsRuleWalker.h
+++ b/layout/style/nsRuleWalker.h
@@ -56,16 +56,16 @@ public:
     mCurrent = aNode;
   }
 
+protected:
   void DoForward(nsIStyleRule* aRule) {
     mCurrent = mCurrent->Transition(aRule, mLevel, mImportance);
     NS_POSTCONDITION(mCurrent, "Transition messed up");
   }
 
+public:
   void Forward(nsIStyleRule* aRule) {
-    // We'd like to assert that this is not a StyleRule, but unfortunately
-    // ResolveStyleByAddingRules can be passed both style and non-style rules.
-    // NS_PRECONDITION(!nsRefPtr<mozilla::css::StyleRule>(do_QueryObject(aRule)),
-    //                 "Calling the wrong Forward() overload");
+    NS_PRECONDITION(!nsRefPtr<mozilla::css::StyleRule>(do_QueryObject(aRule)),
+                    "Calling the wrong Forward() overload");
     DoForward(aRule);
   }
   void Forward(mozilla::css::StyleRule* aRule) {
@@ -73,6 +73,12 @@ public:
     mCheckForImportantRules =
       mCheckForImportantRules && !aRule->GetImportantRule();
   }
+  // ForwardOnPossiblyCSSRule should only be used by callers that have
+  // an explicit list of rules they need to walk, with the list
+  // already containing any important rules they care about.
+  void ForwardOnPossiblyCSSRule(nsIStyleRule* aRule) {
+    DoForward(aRule);
+  }
 
   void Reset() { mCurrent = mRoot; }
 
diff --git a/layout/style/nsStyleSet.cpp b/layout/style/nsStyleSet.cpp
index 86495bbe574..b72aa79c466 100644
--- a/layout/style/nsStyleSet.cpp
+++ b/layout/style/nsStyleSet.cpp
@@ -943,7 +943,7 @@ nsStyleSet::ResolveStyleForRules(nsStyleContext* aParentContext,
   // matter.
   ruleWalker.SetLevel(eDocSheet, PR_FALSE, PR_FALSE);
   for (PRInt32 i = 0; i < aRules.Count(); i++) {
-    ruleWalker.Forward(aRules.ObjectAt(i));
+    ruleWalker.ForwardOnPossiblyCSSRule(aRules.ObjectAt(i));
   }
 
   return GetContext(aParentContext, ruleWalker.CurrentNode(), nsnull,
@@ -964,7 +964,7 @@ nsStyleSet::ResolveStyleByAddingRules(nsStyleContext* aBaseContext,
   // matter.
   ruleWalker.SetLevel(eDocSheet, PR_FALSE, PR_FALSE);
   for (PRInt32 i = 0; i < aRules.Count(); i++) {
-    ruleWalker.Forward(aRules.ObjectAt(i));
+    ruleWalker.ForwardOnPossiblyCSSRule(aRules.ObjectAt(i));
   }
 
   nsRuleNode *ruleNode = ruleWalker.CurrentNode();
@@ -973,7 +973,7 @@ nsStyleSet::ResolveStyleByAddingRules(nsStyleContext* aBaseContext,
   if (aBaseContext->GetStyleIfVisited()) {
     ruleWalker.SetCurrentNode(aBaseContext->GetStyleIfVisited()->GetRuleNode());
     for (PRInt32 i = 0; i < aRules.Count(); i++) {
-      ruleWalker.Forward(aRules.ObjectAt(i));
+      ruleWalker.ForwardOnPossiblyCSSRule(aRules.ObjectAt(i));
     }
     visitedRuleNode = ruleWalker.CurrentNode();
   }