Packs ECMAScript/CommonJs/AMD modules for the browser. Allows you to split your codebase into multiple bundles, which can be loaded on demand. Supports loaders to preprocess files, i.e. json, jsx, es7, css, less, ... and your custom stuff.
The easiest way to write conditional CSS and/or JavaScript based on device operating system (iOS, Android, Blackberry, Windows, Firefox OS, MeeGo, AppleTV, etc), orientation (Portrait vs. Landscape), and type (Tablet vs. Mobile).