We are trying to use jsdom for our unit tests for React core (http://facebook.github.io/react/).

Unfortunately, the web components spec is not natively supported by jsdom, and the webcomponents.js polyfill does not run on jsdom. This issue requests the addition of WebComponent support (custom elements, shadow dom, html imports, etc).