Sync HTML element interfaces A-B with the spec

Source/core/html/HTMLButtonElement.idl

Index: Source/core/html/HTMLButtonElement.idl
diff --git a/Source/core/html/HTMLButtonElement.idl b/Source/core/html/HTMLButtonElement.idl
index 5f1c1686ef7f18b3b2ae782aff5dd56e6591ab81..e4774c7c1d9f2a48c42b6654ccfed07d345e8540 100644
--- a/Source/core/html/HTMLButtonElement.idl
+++ b/Source/core/html/HTMLButtonElement.idl
@@ -18,10 +18,12 @@
  * 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;
@@ -30,12 +32,14 @@ interface HTMLButtonElement : HTMLElement {
     [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;