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In web design, the Z-index is a CSS property that determines the stacking order of elements when they overlap in the same visual space, to be precise along the z-axis, which is the dimension perpendicular to the screen that determines depth (while the x- and y-axes define horizontal and vertical position, respectively, as in Cartesian coordinates in mathematics).

Elements with a higher Z-index value will be displayed above elements with a lower Z-index value, and will thus be in the foreground compared to the others, which will remain in the background. In this way, you can control which elements are more prominent, for example in a drop-down menu or in a modal window.

Unless specified, elements have a default z-index value of 0, and the stacking order follows the order of the HTML code, with subsequent elements stacked on top of previous ones. The z-index property can only be used on positioned elements, that is, those with a positioning property other than ‘static’, such as ‘relative’, ‘absolute’, ‘fixed’ or ‘sticky’.

Proper handling of z-index is critical for creating complex layouts and interactive user interfaces, particularly when working with drop-down menus, modals, tooltips, and other elements that must appear above the normal page content.

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