In the most basic cases, HTML pages can be considered two-dimensional, because text, images, and other elements are arranged on the page without overlapping. In this case, there is a single rendering flow, and all elements are aware of the space taken by others. The z-index attribute lets you adjust the order of the layering of objects when rendering content.... In CSS 2.1, each box has a position...boxes overlap visually. (from CSS 2.1 Section 9.9.1 - Layered presentation...