I think the answer would be have to be yes.
Look at the Flash plugin. It simply takes over the embedded portion of the screen. So I would presume that would be the extent of your CSS plugin.
What sort of cooperation the plugin feature supports? That were what were so important, IMO.
Depends on how the plugin is invoked. Existing examples are invoked after the HTML is parsed and the various embed / object / whatever tags are discovered, by which time, the page has already been rendered.
