Sort of node.cloneNode(true) on descendant only

clone-descendant-deep.js

// will return a cloned version of rootNode descendants
// in itself this function is useless but it can be modified to add custom cloning behaviour
function cloneNodeDeep(rootNode) {
  var node = rootNode;
  var rootClone = document.createDocumentFragment();
  var nodeClone = rootClone;
  while (node) {
      var nextNode = null;
      var firstChild = node.firstChild;
      if (firstChild) {
          nextNode = firstChild;
      } else {
          while (node !== rootNode) {
              var nextSibling = node.nextSibling;
              if (nextSibling) {
                  nextNode = nextSibling;
                  break;
              }
              node = node.parentNode;
              nodeClone = nodeClone.parentNode;
          }
      }

      if (nextNode) {
          var nextNodeClone = nextNode.cloneNode(false);
          if (firstChild) {
              nodeClone.appendChild(nextNodeClone);
          } else { // nextsibling, must append in parent
              nodeClone.parentNode.appendChild(nextNodeClone);
          }

          nodeClone = nextNodeClone;
          node = nextNode;
      } else {
          node = null;
      }
  }
  
  return rootClone;
}