The Yahoo! User Interface Library (YUI) is an 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 YUI Development began in 2005.

It has six types of components: YUI core, utilities, UI controls, CSS components, developer tools, and build tools.

1. The YUI Core is set of tools for event management and DOM manipulation.

2. Utilities are for Animation, Browser History Manager, Connection Manager, Error handling and Cookies.

3. Controls are for AutoComplete, Button, Calendar, Charts Control, Color Picker, Container etc.

YUI components get their look and feel from a visual "skin" that you can modify or replace in your own applications. YUI Library is a great way to calculate an optimized list of script/css includes for your particular implementation.