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 in three dimensions. In addition to their horizontal and vertical positions, boxes lie along a "z-axis" and are formatted one on top of the other. Z-axis positions are particularly relevant when boxes overlap visually.

(from CSS 2.1 Section 9.9.1 - Layered presentation)

This means that CSS style rules allow you to position boxes on layers in addition to the default rendering layer (layer 0). The position on an imaginary z-axis of each layer is expressed as an integer representing the stacking order for rendering. Greater numbers mean closer to the observer. Control the position on this z-axis with the CSS z-index property.

Using z-index appears extremely easy: a single property assigned a single integer number with an easy-to-understand behavior. When z-index is applied to complex hierarchies of HTML elements, its behavior can be hard to understand or predict. This is due to complex stacking rules. In fact, a dedicated section has been reserved in the CSS specification CSS-2.1 Appendix E to explain these rules better.

This guide will try to explain those rules, with some simplification and several examples.