It's responsive Web Design on steroids. Imagine being able to design a site that reacts to the user, their location, the time of day, the traffic. We could deduce and infer many things about a user as they view a site and it would be nice to see the builders of the web embrace catering to users and their needs/experience with more than just layout.
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Reactive web design | Opinion | .net magazine
Dan Donald argues that responsive web design is only scratching the surface of how sites can adapt to meet their users’ requirements