--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -4441,16 +4441,27 @@ NS_IMETHODIMP
 nsDOMWindowUtils::GetIsStyledByServo(bool* aStyledByServo)
 {
   nsIDocument* doc = GetDocument();
   *aStyledByServo = doc && doc->IsStyledByServo();
   return NS_OK;
 }
 
 NS_IMETHODIMP
+nsDOMWindowUtils::GetIsOldStyleSystemEnabled(bool* aResult)
+{
+#ifdef MOZ_OLD_STYLE
+  *aResult = true;
+#else
+  *aResult = false;
+#endif
+  return NS_OK;
+}
+
+NS_IMETHODIMP
 nsDOMWindowUtils::AddToStyloBlocklist(const nsACString& aBlockedDomain)
 {
 #ifdef MOZ_STYLO
   nsLayoutUtils::AddToStyloBlocklist(aBlockedDomain);
 #endif
   return NS_OK;
 }
 

--- a/dom/interfaces/base/nsIDOMWindowUtils.idl
+++ b/dom/interfaces/base/nsIDOMWindowUtils.idl
@@ -1976,16 +1976,24 @@ interface nsIDOMWindowUtils : nsISupport
   /**
    * Whether the current document is styled by Servo's style engine.
    *
    * This calls nsIDocument::IsStyledByServo().
    */
   readonly attribute boolean isStyledByServo;
 
   /**
+   * Whether the old style system is available.
+   *
+   * This is used to skip some tests that make no sense if we only have
+   * Servo's style system.
+   */
+  readonly attribute boolean isOldStyleSystemEnabled;
+
+  /**
    * Add a domain to the existing stylo blocklist.
    *
    * This calls nsLayoutUtils::AddToStyloBlocklist().
    */
   void addToStyloBlocklist(in ACString aBlockedDomain);
 
   /**
    * Remove a domain from the existing stylo blocklist.