Because all the columns are floated, this layout uses overflow:hidden on the .container. This clearing technique forces the .container to understand where the columns end in order to show any borders or background colors you place on the .container. If you have a large element that protrudes outside the .container, it will appear to be cut off. You also won't be able to use negative margins or absolute positioning with negative values to pull elements outside the .container or they will also won't display outside the .container.