Custom elements are a part of the W3C Web Components specification http://w3c.github.io/webcomponents/spec/custom/. They allow you to define and register new HTML tags/elements in your documents. You can then use these tags as regular HTML.
This library polyfills the Custom Elements API on browsers today. It is a barebones fork of the X-Tag core library from Mozilla http://www.x-tags.org/. X-Tags in turn uses the Polymer polyfills from Google http://www.polymer-project.org/.
The aim of this library is to provide a stripped down version of the above polyfills, with zero additional weight.