This is the <body> containing block.

This is the <div> containing block.

This <p> element generates its own block box inside the <div> box. This content is in the <b> element. As an inline element, it generated its own "inline" box within the <p> element's block box.

This content (with the white background) is not contained in a block-level or inline element. For layout purposes it is considered to be in an "anonymous" box.