The following elements (divs) are placed absolutely on this page based on the entire document (browser window). To test this, press F11 on your browser to view full screen. Notice how absolute elements stay in place? You can change this, however, by changing the position element of a parent element to absolute, relative, or fixed.
For example, I can specify the top, bottom, right, and left pixel position of any element. I can also use z-index to set the layer order for the element (which element is on top). Z-index only works when an element is positioned (absolute, relative, or fixed). Highest z-index value goes on top!