third_party/web_platform_tests/WebIDL/valid/idl/nullable.widl

// Extracted from http://dev.w3.org/2006/webapi/WebIDL/ on 2011-05-06
interface MyConstants {
  const boolean? ARE_WE_THERE_YET = false;
};

interface Node {
  readonly attribute DOMString? namespaceURI;
  // ...
};