* This method can only be applied to elements which accept child nodes.
* @param {String} msg (optional) A message to display in the mask
* @param {String} msgCls (optional) A css class to apply to the msg element
* This method can only be applied to elements which accept child nodes.
* @param {String} msg (optional) A message to display in the mask
* @param {String} msgCls (optional) A css class to apply to the msg element