How does z-index work?

The z-index property in CSS is used to control the stacking order of overlapping elements. Its functionality can be summarized as follows:

1. The z-index property only works on positioned elements (elements with a position value of relative, absolute, or fixed) and has no effect on static elements.
2. The higher the z-index value of an element, the closer it is to the user in terms of its position in the hierarchy. Elements with a higher z-index value will overlay those with a lower z-index value.
3. When elements overlap, the z-index property can be used to specify the hierarchy of elements. Elements with a higher z-index value will be positioned above elements with a lower z-index value.
4. If two elements have the same z-index value, their stacking order is determined by their order in the HTML document, with the later appearing element placed above the earlier one.
5. If elements do not have a defined z-index property, their z-index value defaults to auto, which means their layering is determined by their order in the HTML document.
6. Apart from integer values, the z-index property can also take negative values, where a negative value indicates that an element is positioned below the normal flow. The lower the negative value, the lower the element is positioned.

In conclusion, the z-index property controls the stacking order and overlapping effects of elements by setting their layering relationships.