The <body> element is the initial containing block. It contains all of
the other elements that will appear on this page, such as the following block-level elements:
This heading is contained within the <body> containing block.
So is this <h2> element.
So is this <h3> element.
This <p> element is also contained within the <body> containing block.
As a rule, elements such these derive their position and
their width from their containing blocks
This <div> element is also contained within the initial containing block.