How to place a box in the middle of the browser window?

Sometime developers are really afraid to do a silly thing, like how to center a div or how to middle align a custom pop-up or how to middle align an image. Even they looking for HTML expert help or spent lots of time on browsing to find out the solution. But can you imagine that a single class can do all the above things.

Suppose you want to use a message box in the middle of the browser window. Then see how you will do it with a single class for all monitor resolution.