According to informit:
Look out, Dreamweaver. Consider Microsoft Expression Web Designe
r as Dreamweaver’s new rival....
FrontPage is out of the race. Microsoft has gotten serious about targeting creative professionals including Web designers and user interface developers, with its Expression suite—consisting of Graphic Designer, Interactive Designer, and Web Designer.
With Interactive Designer (previously known by its code name Sparkle), designers and developers build user interfaces for applications based on the Windows Presentation Foundation (WPF) and extensible application markup language (XAML).
What designers might not realize is that Microsoft finally drank the Kool-Aid. The Expression Web Designer application walks the Web standards walk. One caution: Web Designer currently only supports ASP.NET. Microsoft built the ASP.NET platform; it isn’t a surprise that Expression Web Designer was designed to support that platform. This is obviously a drawback for those designers who work with PHP, JSP, and other non-ASP.NET platforms, making it difficult for Microsoft to expand its reach beyond the ASP.NET users."