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.