Question: When Browser completes formatting a page using CSS and HTML etc.. Where will be all the output will be stored..? Is there any virtual location where it will store..  

Can we access that content in the background..?

Answer: 
It's stored in the DOM, which is the Document Object Model, which is a cross-platform and language agnostic convention that stores these objects that austincheney spoke about, which can then be manipulated by a scripting language like JavaScript.

The Wikipedia entry is a good place to start reading about it: <http://en.wikipedia.org/wiki/Document_Object_Model>

If you'd like to learn more about manipulating it, you can have a look at <http://www.codecademy.com/> which lets you learn JavaScript basics interactively.
To add to austincheneys accurate summary (+1) this page explains the text nodes you content is in within the DOM he refers to: [w3C - Traversing the DOM](http://www.w3.org/community/webed/wiki/Traversing_the_DOM)

Particularly, look at the diagram halfway down the page.