Usually HTML pages can be considered two-dimensional, because text, images and other elements are arranged on the page without overlapping. 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.
It 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
Using 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 even unpredictable. 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.
Note of the author: Thanks to Wladimir Palant and Rod Whiteley for the review.
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