Clipboard API: Default content in copy/cut handlers

I've noticed that the way that drag-and-drop processing model is written,
the default content that would be in the drag data store is available in
the dragstart event.
http://www.whatwg.org/specs/web-apps/current-work/multipage/dnd.html#drag-and-drop-processing-model
specifies
that the drag data store is populated before dispatching the dragstart
event.

Would it make sense to do something similar for
http://dev.w3.org/2006/webapi/clipops/#processing-model? Right now, as I
read it, we don't populate the clipboard until after the copy/cut event has
been cancelled. It'd be nice to make it consistent with drags... the main
problem is I'm not sure if this will break compatibility with sites that
didn't expect this.

Daniel