Finding rendered HTML element positions using WebKit (or Gecko)
lxml isn't going to help you at all. It isn't concerned about front-end rendering at all.
To accurately work out how something renders, you need to render it. For that you need to hook into a browser, spawn the page and run some JS on the page to find the DOM element and get its attributes.
It's totally possible but I think you should start by looking at how website screenshot factories work (as they'll share 90% of the code you need to get a browser launching and showing the right page). 
You may want to still use lxml to inject your javascript into the page.
I would like to get the dimensions (coordinates) for all the HTML elements of a webpage as they are rendered by a browser, that is the positions they are rendered at. For example, (top-left,top-right,bottom-left,bottom-right)
Could not find this in lxml. So, is there any library in Python that does this? I had also looked at Mechanize::Mozilla in Perl but, that seems difficult to configure/set-up.
I think the best way to do this for my requirement is to use a rendering engine - like WebKit or Gecko.
Are there any perl/python bindings available for the above two rendering engines? Google searches for tutorials on how to "plug-in" to the WebKit rendering engine is not very helpful.