diff --git a/dom/webidl/MutationObserver.webidl b/dom/webidl/MutationObserver.webidl
index 91e499d72797..23297eac556b 100644
--- a/dom/webidl/MutationObserver.webidl
+++ b/dom/webidl/MutationObserver.webidl
@@ -8,16 +8,25 @@
  */
 
 interface MutationRecord {
+  [Constant]
   readonly attribute DOMString type;
   // .target is not nullable per the spec, but in order to prevent crashes,
   // if there are GC/CC bugs in Gecko, we let the property to be null.
+  [Constant]
   readonly attribute Node? target;
+  [Constant]
   readonly attribute NodeList addedNodes;
+  [Constant]
   readonly attribute NodeList removedNodes;
+  [Constant]
   readonly attribute Node? previousSibling;
+  [Constant]
   readonly attribute Node? nextSibling;
+  [Constant]
   readonly attribute DOMString? attributeName;
+  [Constant]
   readonly attribute DOMString? attributeNamespace;
+  [Constant]
   readonly attribute DOMString? oldValue;
 };