Around 8 years ago I was on a project where my boss told me the web app I'd be working on was really slow, and they needed me to help them speed it up. When I took a look at it, I discovered the app was using a multitude of libraries with overlapping functionality - jQuery, ExtJS, Mochikit, YUI, and I think a few others. Every previous developer had simply included their library of choice when working on the app, and it had made the app overly bloated (and complicated). Definitely a good sign that browser APIs and their polyfills have become good enough for standard web app development.