HTML's `required` attribute on form elements doesn't parse correctly

Created by: travisleithead

        interface HTMLSelectElement : HTMLElement {
          attribute DOMString name;
          attribute boolean required;
          attribute unsigned long size;

The WebIDL parser seems to skip right over the 'required' IDL attribute. This prevents proper linking through bikeshed.

Might have something to do with 'required' dictionary members being a WebIDL keyword?