Re: Presentation API changes proposal

----- Original Message -----

> Promise requestShow(optional DOMString url = "about:blank" | DOMElement elem
> , optional boolean infinitePlay = false);

> In case the first parameter is a DOM Element, The DOM Subtree starting by
> this element will be displayed on the second screen. Any manipulation in the
> DOM subtree on the original document (first screen device) results in
> updating the copy on the second screen (MutationObservers[1] is one possible
> way to observe changes). Updates can be sent for example over a
> RPCDataChannel.

I'm a little worried about allowing arbitrary DOM subtrees. Other DOM elements can float over the assumed "viewport" of the subtree, making it hard to figure out what exactly to mirror. I also think it could be a bit more difficult to implement. 

On the other hand, I like the idea of using the window as a root. Either the hidden secondary window or the current main window (the tab sharing concept).