--- a/layout/style/ServoCSSRuleList.cpp
+++ b/layout/style/ServoCSSRuleList.cpp
@@ -66,17 +66,17 @@ ServoCSSRuleList::GetRule(uint32_t aInde
         break;
       }
       case nsIDOMCSSRule::MEDIA_RULE:
       case nsIDOMCSSRule::FONT_FACE_RULE:
       case nsIDOMCSSRule::KEYFRAMES_RULE:
       case nsIDOMCSSRule::NAMESPACE_RULE:
         // XXX create corresponding rules
       default:
-        NS_ERROR("stylo: not implemented yet");
+        NS_WARNING("stylo: not implemented yet");
         return nullptr;
     }
     ruleObj->SetStyleSheet(mStyleSheet);
     rule = CastToUint(ruleObj.forget().take());
     mRules[aIndex] = rule;
   }
   return CastToPtr(rule);
 }