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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. 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 normal rendering layer (layer 0). The Z position of each layer is expressed as an integer representing the stacking order for rendering. Greater numbers mean closer to the observer. Z position can be controlled with the CSS z-index 特性。

使用 z-index appears extremely easy: a single property, assigned a single integer number, with an easy-to-understand behaviour. However, when z-index is applied to complex hierarchies of HTML elements, its behaviour 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 article will try to explain those rules, with some simplification and several examples.

1. Stacking without the z-index property : The stacking rules that apply when z-index is not used.
2. Stacking with floated blocks : How floating elements are handled with stacking.
3. Using z-index : How to use z-index to change default stacking.
4. The stacking context : Notes on the stacking context.
5. Stacking context example 1 : 2-level HTML hierarchy, z-index on the last level
6. Stacking context example 2 : 2-level HTML hierarchy, z-index on all levels
7. Stacking context example 3 : 3-level HTML hierarchy, z-index on the second level