docs: function _import(urls, callback) {
    if (urls && urls.length) {
        var frag = document.createDocumentFragment();
        for (var i=0, l=urls.length, url, link; (i<l) && (url=urls[i]); i++) {
          link = document.createElement('link');
          link.rel = 'import';
          link.href = url;
          frag.appendChild(link);
        }
        importElements(frag, callback);
    } else if (callback) {
      callback();
    }
  }