Currently there is a snapshot of the final rendered page.
It would be good to have a snapshot of the first render as well.
it would also be good to have counters for the number of reflows and repaints that take place between the initial render and the final render.
The purpose of the initial render screenshot would be to encourage, and display the results of, using HTTP 1.1 chunked transfer and flushing the head of the HTML document (and maybe a banner section) early on to kickstart downloading of elements before the rest of the HTML page has been computed.
The counters for reflows and repaints will give an indication as the efficiency of your CSS and JS in interacting with the DOM and browser rendering engine.
This site also has a very pretty way of visualising/fingerprinting your page’s DOM that would make a nice souvenir of a visit to webpagetest.