Sync HTML element interfaces A-B with the spec

Source/core/html/HTMLButtonElement.idl

 /*
  * Copyright (C) 2006, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  *
  * 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.
  */
 
+// https://html.spec.whatwg.org/#htmlbuttonelement
+
 interface HTMLButtonElement : HTMLElement {
     [Reflect] attribute boolean autofocus;
     [Reflect] attribute boolean disabled;
-    [ImplementedAs=formOwner] readonly attribute HTMLFormElement form;
+    [ImplementedAs=formOwner] readonly attribute HTMLFormElement? form;
     [Reflect, URL] attribute DOMString formAction;
     attribute DOMString formEnctype;
     attribute DOMString formMethod;
     [Reflect] attribute boolean formNoValidate;
     [Reflect] attribute DOMString formTarget;
     [Reflect] attribute DOMString name;
     attribute DOMString type;
     [Reflect] attribute DOMString value;
+    // FIXME: attribute HTMLMenuElement? menu;
 
     readonly attribute boolean willValidate;
     readonly attribute ValidityState validity;
     readonly attribute DOMString validationMessage;
     boolean checkValidity();
     boolean reportValidity();
+    // FIXME: The error argument should not be nullable.
     void setCustomValidity([TreatUndefinedAs=NullString] DOMString? error);
 
     readonly attribute NodeList labels;
 };