Document order is pretty simple. Block-level elements will appear in the order they appear in the HTML, and take as much horizontal space as they can, and as much vertical space as they need.
The only restriction on this page is a container div is being used to limit the content to 70% of the browser width, with margins being used to center. To see this more distinctly, use the anchors below to turn backgrounds on and off (or use a web development tool).