When making a website today, there are many of pieces that need to work together. You need to create the WebForm in HTML, the codebehind in C# or VB, the stylesheets in CSS, the scripts in Javascript and the animations in Flash / ActionScript. Just making the form look just right by switching back and forth between the code editor and browser, determining which shows up in Internet Explorer but not Firefox. Internet Explorer handles bugs in Javascript by saying "Object expected" on the wrong line in the wrong file. Even the codebehind is messy because it's very difficult to separate the view from the application logic.
Silverlight will make the web application look better, and will be easier to create. WPF applications are the next evolution in user interfaces; Expression Blend is a powerful way to create beautiful applications. You don't have to worry about time-consuming cross-browser CSS issues because the Silverlight plugin will ensure that things are consistent across browsers and operating systems. This makes it easy to separate the code affecting the view into the XAML file and the application logic on the server. Consider Silverlight's ability to stream audio and video and you can get an idea of what's possible. While it is still in beta some people are already showing us how powerful Silverlight really is.
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