In CSS, what is the purpose of the z-index property?

The z-index property in CSS is primarily used to specify the stack order of elements. When elements overlap, the z-index determines which element appears in front of the others. Elements with a higher z-index value will cover those with a lower value, allowing for precise control over how elements are visually layered on the page.

The z-index only works on positioned elements, meaning those elements that have a position property set to relative, absolute, fixed, or sticky. By assigning different z-index values to these positioned elements, developers can create complex layouts where certain items, such as modals, dropdowns, or images, can overlay others as desired. This capability is essential for creating a visually appealing and functional user interface in web applications.