ed template content and returns a document fragment
       * containing the cloned dom.
       *
       * The template is parsed (once and memoized) using this library's
       * template parsing features, and provides the following value-added
       * features:
       * * Adds declarative event listeners for `on-event="handler"` attributes
       * * Generates an "id map" for all nodes with id's under `$` on returned
       *   document fragment
       * * Passes template info including `content` back to 