Collapsing margin
The expression collapsing margins means that adjoining margins (no non-empty content, padding or border areas or clearance separate them) of two or more boxes (which may be next to one another or nested) combine to form a single margin. In CSS 2.1, horizontal margins never collapse.
Anything that stops adjoining margins from “touching” each other will prevent them to collapse.
Style the yellow box with:
border-color: transparent