The actual License of Yahoo YUI
are here :
http://developer.yahoo.com/yui/license.html

Software License Agreement (BSD License)
Copyright (c) 2008, Yahoo! Inc.
All rights reserved.
Redistribution and use of this software in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

Redistributions of source code must retain the above copyright notice, this list of conditions and the
following disclaimer.
Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
Neither the name of Yahoo! Inc. nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission of Yahoo! Inc.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Sources of Intellectual Property Included in the YUI Library
Where not otherwise indicated, all YUI content is authored by Yahoo! engineers and consists of Yahoo!-owned intellectual property. YUI is issued by Yahoo! under the BSD license above. In some specific instances, YUI will incorporate work done by developers outside of Yahoo! with their express permission. In these instances, copyright is retained by the author; however, in the context of the YUI library it is issued under the BSD license.

Below is the explicit list of external (non-Yahoo) sources of intellectual property in YUI:

Douglas Crockford's JSON parsing and stringifying methods: In the JSON Utility, Douglas Crockford's JSON parsing and stringifying methods are adapted from work published at JSON.org. The adapted work is in the public domain.
Robert Penner's animation-easing algorithms: In the Animation Utility, YUI makes use of Robert Penner's algorithms for easing.
