Generally, you would find that different page rendering engines do render the html in their own way and the results will differ.
The thing is that if you stick to any concrete browser engine, what you are to do is somehow bringing this engine into your project and using engine's interface to retrieve these coordinates. Kind of a tough task though, simply because you'll have to read a lot of documentation and crawl through thousands of files.
I think that right approach would be posting this task in some place, that is specific for the page rendering engine you've chosen. (gecko/webkit/...)
If you prefer sticking to something MS-specific, guess it's gonna be easier, but can't help you with something like class names or code chunks that you want to see. Probably somebody else could guide you in this case.