Development of YUI 3.0, the latest version of Yahoo's User Interface toolset and widgets for Web development, is now available as a preview release. YUI has long been a popular choice for client-side Web development, both because of its liberal BSD license, and because of the number of features that the library includes. YUI 3.0, when it is released, will offer a variety of new JavaScript classes and widgets. However, this will come at a price, namely compatibility with YUI 2.x APIs.