It's both. This is a slightly outdated image, but it describes it pretty well: http://4.bp.blogspot.com/-rPbEdmEpHWA/UeW4Xqmg0oI/AAAAAAAAAx0/33O2gWoYMgI/s1600/p

It's both. This is a slightly outdated image, but it describes it pretty well: http://4.bp.blogspot.com/-rPbEdmEpHWA/UeW4Xqmg0oI/AAAAAAAAAx0/33O2gWoYMgI/s1600/polymer-architecture.jpg http://4.bp.blogspot.com/-rPbEdmEpHWA/UeW4Xqmg0oI/AAAAAAAAAx... Basically, there's a layer of polyfills called the Platform that will disappear as browsers implement key web platform features natively. On top of that is the main Polymer library, which is a small sugaring library that makes using the individual Platform polyfills together a lot easier. On top of that are a couple different sets of both UI and non-UI web components built using Polymer, namely core-elements and paper-elements.