Css Playground and Dom Tree

Unable to see DOM child number/index

Thank you for the extension. It works really nice and shows the heirarchy to get clear idea about the DOM tree and where the element stays exactly in the tree.
Though I am not able to see the index of the DOM element as per the screenshot on the main extension page. Green colour "#childnumber" property. Is it possible to avail that feature or any idea why I am not able to see that property?
Thnx.