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The Yahoo! next-generation JavaScript and CSS library, is a big step forward.
YUI team has improved the library; syntax is enhanced, API is more flexible and easy to use.

Featured Components:

1. DataSource: YUI’s data abstraction layer provides a standard interface into data sets, regardless of the data’s origin (local, XHR, XSS, etc.) and format (JSON, XML, CSV, etc.);
2. ImageLoader: ImageLoader allows you to defer the loading of images that aren’t in the viewport when the page paints, throttling bandwidth usage and improving performance;
3. History: The History component gives you control of the brower’s back button within the context of a single-page web application;
4. StyleSheet: StyleSheet makes it easy to create and modify CSS rules on the fly, allowing you to dynamically style page elements with fewer repaints.

More:
http://developer.yahoo.com/yui/3/