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

Source/core/html/HTMLInputElement.idl

 /*
  * Copyright (C) 2006, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  * Copyright (C) 2012 Samsung Electronics. All rights reserved.
  *
  * 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.
  *
@@ skipping 11 matching lines @@
 interface HTMLInputElement : HTMLElement {
     [Reflect] attribute DOMString accept;
     [Reflect] attribute DOMString align;
     [Reflect] attribute DOMString alt;
     [Reflect] attribute DOMString autocomplete;
     [Reflect] attribute boolean autofocus;
     [Reflect=checked] attribute boolean defaultChecked;
     attribute boolean checked;
     [Reflect] attribute DOMString dirName;
     [Reflect] attribute boolean disabled;
-    readonly attribute HTMLFormElement form;
+    [ImplementedAs=formOwner] readonly attribute HTMLFormElement form;
     // The 'files' attribute is intentionally not readonly.
     // https://www.w3.org/Bugs/Public/show_bug.cgi?id=22682
     attribute FileList files;
     [Reflect, URL, PerWorldBindings, ActivityLogging=SetterForIsolatedWorlds] attribute DOMString formAction;
     [CustomElementCallbacks] attribute DOMString formEnctype;
     [CustomElementCallbacks] attribute DOMString formMethod;
     [Reflect] attribute boolean formNoValidate;
     [Reflect] attribute DOMString formTarget;
     [CustomElementCallbacks] attribute unsigned long height;
     attribute boolean indeterminate;
@@ skipping 50 matching lines @@
     // Non-standard attributes
     [Reflect, RuntimeEnabled=DirectoryUpload] attribute boolean webkitdirectory;
     [Reflect] attribute boolean incremental;
     [Conditional=INPUT_SPEECH, Reflect, RuntimeEnabled=SpeechInput] attribute boolean webkitSpeech;
     [Conditional=INPUT_SPEECH, Reflect, RuntimeEnabled=SpeechInput] attribute boolean webkitGrammar;
     [Conditional=INPUT_SPEECH] attribute EventHandler onwebkitspeechchange;
 
     // See http://www.w3.org/TR/html-media-capture/
     [Conditional=MEDIA_CAPTURE, Reflect] attribute boolean capture;
 };