9-year veteran YUI is discontinued “in order to focus our efforts on this new technology landscape”
Basically due to lack of attention internally and usage by developers externally forced Yahoo to make a tough decision—move forward without a strong community or officially close shop.
This long-winded sometimes-rambling announcement was made Aug 29th, 2014 by Julien Lecomte, Director of Engineering, Yahoo Presentation Technologies.
YUI has some really good widgets that have been hard to rival including their calendar widget. They were also the first to have a JSON parse/stringify that was fully spec compliant.
I use YUI docs and will continue to do so. And maybe some of their other good stuff.
Read more from the source: Yahoo Engineering