We also had a discussion about MSHTML versus the rich text rendering engine in Avalon. I had wondered whether Microsoft might rewrite MSHTML in managed code, or else create a Longhorn web browser that converts HTML to Avalon rich text. Chris said no. He said that MSHTML is a complex body of code which does an extraordinary job of rendering even malformed HTML. Since the Internet works on the assumption that this kind of web page renders successfully, and since there are so many quirks and workarounds in MSHTML, Microsoft's developers did not dare to replace it. I hope I'm representing Chris's comments accurately. In essence, it suggests that IE will remain broadly as-is, well, for ever. For richer Internet applications, you are meant to write Windows Forms or in Longhorn Avalon clients for web services. I do find this surprising. It suggests that IE will gradually lose ground to Mozilla, Safari, Opera and others. It also means that we are not going to get a managed code HTML or XHTML rendering component. Of course the rich text engine in Avalon is still a big step forward, and will be easier to work with than either MSHTML or old-style RTF.
[via Tim Anderson's ITWriting]
For Longhorn, you want to do a UI delivered from a server, you use XAML. You do not use anything else. Legacy operations will of course be supported by the continued presense of IE.