From this site, I learned that this way can center an element. But why? What is the sequence of the elements calculating the position? How does it exactly work? Which rule has higher priority to another? What is the exact meaning of "bounding box"?
.Absolute-Center {
margin: auto;
position: absolute;
top: 0; left: 0; bottom: 0; right: 0;
width: 100px;//exact width
height: 100px;//exact height
}