DocumentFragment is a “lightweight” or “minimal” Document object.
nsIDOMDocumentFragment is used in situations where the Document
interface can potentially be a heavyweight interface.
For more information on this interface please see
http://www.w3.org/TR/DOM-Level-2-Core/
Return nodes that match a given CSS selector.
@see http://dev.w3.org/2006/webapi/selectors-api/