YUI is a free, open source JavaScript and CSS library for building richly interactive web applications. YUI's lightweight core and modular architecture make it scalable, fast, and robust.YUI's intuitive and well-documented API takes you from basic DOM handling to building performant and maintainable applications on desktop browsers, mobile devices, and servers.

The Yahoo User Interface (YUI) Library is a set of utilities and controls, written in JavaScript, for building richly interactive web applications using techniques such as DOM scripting, DHTML and AJAX. The YUI Library also includes several core CSS resources. All components in the YUI Library have been released as open source under a BSD license and are free for all uses. Download the entire library, its documentation and examples from Sourceforge.The library's developers blog at the YUI Blog and the YUI Library community exchanges ideas at ydn-javaScript on Yahoo Groups.