z-index represents the order on the z-plane in which an element appears. An element with higher stack order is always in front of another element with lower stack order. It is available in most of the GUI frameworks (both Web and Desktop).

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Z-index is an inherently tricky thing, and maintaining z-index order in a complex layout is notoriously difficult. With different stacking orders and contexts, keeping track of them as their numbers increase can be hard — and once they start to spread across CSS files, forget about it! Because z-index can make or break a UI element’s visibility and usability, keeping your website’s UI in working order can be a delicate balance.