Yahoo! recommends YUI 3. Learn about YUI 3 AutoComplete on YUILibrary.com.
AutoComplete is a powerful interaction pattern that leverages both the power of the server and the power of the client. Examples here show how to use server-side completion engines, client-side completion engines, and rich visualizations within your suggestion container.
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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