Following is the interface for a clipboard event:

interface ClipboardEvent : Event {
  constructor(DOMString type, optional ClipboardEventInit eventInitDict = {});
  readonly attribute DataTransfer? clipboardData;
};

To ensure spec is not open to interpretation by UAs, an explicit mention that clipboadData (DataTransfer object) would be null in clipboardChange event:

• 'clipboardchange' event would only indicate that system clipboard has changed. And clipboardData for this event will be set to null (Web developers are expected to use Async clipboard read API to get the clipboard data.). This ensures the lightweight nature of the event.
• Web developers should continue to use Async clipboard read API to get the clipboard data.