The Yahoo! User Interface Library (YUI) is a discontinued open-source JavaScript library for building richly interactive web applications using techniques such as Ajax, DHTML, and DOM scripting. YUI includes several core CSS resources. It is available under a BSD License.Development on YUI began in 2005 and Yahoo! properties such as My Yahoo! and the Yahoo! front page began using YUI in the summer of that year. YUI was released for public use in February 2006. It was actively developed by a core team of Yahoo! engineers.