I was wondering, if you had two divs that were postioned at the same place, which on would be on top?

if you are not using z-index, then the "top" one will be the last one seen in your coding.

the order of your coding is the order of the appearance.

Ok, thanks. How do I use z-index?

To use z-index:
http://www.w3schools.com/css/css_positioning.asp

z-index is an attribute of <div>.
By the way, how can we center that <div> on the webpage ??

Thanks.