Moving CSS boxes to center in webpage once browser size is big enough?

I've noticed on many websites from Facebook to stackoverflow that most elements have a fixed hight and width and are placed at a certain distance from the left border of the page. However, once the page is stretched, all the elements move together towards a more central position within the webpage. How could this be executed in Html and CSS with just a couple of box elements?

any help is greatly appreciated Thanks in advance.