Yahoo! recommends YUI 3. Learn about the YUI 3 Global Object on YUILibrary.com.
The YUI Loader provides a client-side mechanism for loading YUI components and their dependencies. It can be used to add additional components to a page on which some YUI components already exist or to structure the loading of scripts and CSS files for an entire page.
All YUI 2.x users should review the YUI 2.8.2 security bulletin, which discusses a vulnerability present in YUI 2.4.0-2.8.1.

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