CSS z-index
CSS z-index is a property that controls the stacking order of positioned elements (those with position values like relative, absolute, fixed, or sticky) along the z-axis (depth) on a web page. It determines which elements appear in front of or behind others when they overlap, with higher values bringing elements to the front and lower values sending them to the back. This property is essential for creating layered layouts, modals, dropdown menus, and complex visual hierarchies in web design.
Developers should learn and use CSS z-index when building interfaces with overlapping elements, such as modal dialogs, tooltips, navigation menus, or custom UI components that require precise layering control. It is particularly crucial for ensuring that interactive elements remain accessible and visible above other content, and for managing visual depth in animations or parallax scrolling effects. Without proper z-index management, elements may unintentionally obscure each other, leading to usability issues.