YUI

YUI (the Yahoo User Interface Library) shipped in 2006 and was for a few years one of the most ambitious front-end frameworks around. It offered a complete kit: DOM and event handling, animation, ready-made widgets, and the influential CSS reset (reset.css) and grids that many sites copied.

Yahoo also published its "Graded Browser Support" — a matrix that formalized how much support each browser should get, an approach that became an industry standard. YUI competed with jQuery, Prototype and Dojo but lost ground as the JavaScript ecosystem moved to Node, npm and modules. In 2014 Yahoo announced that development had stopped; the framework became one of the clearest casualties of how fast front-end tooling turned over.