A sequence of pages follows a page master.
A page master defines the basic layout of a page and the regions on the page.
There is a "simple page master" built-in with the following regions:
region-before - The header of the page.
region-after - The footer of the page.
region-start - The "left margin" region.
region-end - The "right margin" region.
region-body - The "right margin" region.
We'll "flow" content into these regions.