Only 'form associated' elements have a potential form owner.

Source/core/html/HTMLFieldSetElement.idl

 /*
  * Copyright (C) 2006 Apple Computer, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  */
 
 interface HTMLFieldSetElement : HTMLElement {
     [Reflect] attribute boolean disabled;
-    readonly attribute HTMLFormElement form;
+    [ImplementedAs=formOwner] readonly attribute HTMLFormElement form;
     [Reflect] attribute DOMString name;
 
     readonly attribute DOMString type;
 
     readonly attribute HTMLCollection elements;
 
     readonly attribute boolean         willValidate;
     readonly attribute ValidityState   validity;
     readonly attribute DOMString       validationMessage;
     boolean  checkValidity();
     void     setCustomValidity([TreatNullAs=NullString, TreatUndefinedAs=NullString] DOMString error);
 };