Z-Index

Z-Index is a CSS property that controls the stacking order of elements on a webpage along the z-axis (which is perpendicular to the screen). When elements overlap visually, the one with the higher z-index appears in front.

Example:

.box1 {
  position: absolute;
  z-index: 10;
}
.box2 {
  position: absolute;
  z-index: 20;
}

In the example above, .box2 will render on top of .box1 because it has a higher z-index value.

Z-index only works on elements that have a position value other than static—such as relative, absolute, or fixed. Without a non-static position, the property won’t have any effect.

Use cases include:

• Layering modals or popups over content

• Ensuring dropdown menus appear above other elements

• Managing multiple overlapping elements in interactive layouts

It's worth noting that z-index values are relative within stacking contexts. Creating a new stacking context (e.g., via position, opacity, or transform) can isolate z-index behavior, which sometimes leads to tricky bugs in complex UIs.

Mastering z-index is key for achieving precise and predictable visual hierarchies in web design.

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