The sun comes out and shines on us. Heres what is now possible, and straightforward, that a few hours ago seemed impossible :
We can add an Internet Explorer pane to the notepad. We can use a tabbed window, or two panes on one big page, or any other layout. If the XML in the notepad is legal and includes the right xsl-stylesheet line, we can write the XML to a file and tell the Internet Explorer pane to navigate to it. We can do that any time we think is the right timewe could do it on every character the user types if we want to.
We might even be able to find a way to avoid writing the file out. It looks like the browser COM object may be able to accept its input as a string. Or the customer might decide that every time he saves, we should refresh the browser pane. We could even create a separate little window, with a browser pane in it, and a FileWatcher watching for file updates. We could use asynchronous delegates (whatever those are) to make the panes or windows communicate but run in separate threads! And every one of those ideas is close enough to understandable that we can estimate it, giving our customer a fair chance to decide if he wants to buy it.
The skys the limit, the world is our oyster, were in the money. With a couple of hours work, weve taken a problem from could be impossible and broken it down into a fairly manageable number of options that we understand reasonably well. Time well spent.