From the Expression website:
“The web is now about applications as well as traditional web sites, and this requires a new set of tools. Microsoft is committed to offering a unified approach to focus on web design and development features in Microsoft Visual Studio 2012.
Expression Web is now available as a free download from the Microsoft Download Center, and no new versions will be developed. Customers who previously purchased Expression Web will receive support through the established support lifecycle.”
Together with Expression Web, the SuperPreview service will come to an end, too:
“Expression SuperPreview Remote Beta will continue running as a service through June 30, 2013”