Z-index

The z-index property in CSS is a tool that allows controlling the stacking of elements relative to each other along the Z-axis, or "depth." With it, we can determine which element should be displayed on top and which at the bottom.

What is z-index?

The z-index property determines how elements overlap each other when they occupy the same space on the screen. It is an integer value (can be positive, negative, or zero) that determines how far "forward" or "backward" an element is placed relative to other elements.

How Does z-index Work?

The z-index property only works with elements that have their position set to relative, absolute, fixed, or sticky.

Example:

1div {
2  position: absolute;
3  z-index: 1; /* Element will be above other elements with lower z-index value */
4}

Use Cases

The z-index property is particularly useful when you want certain elements to be visible above others, just as stars can be visible above planets in a model of the cosmos.

Navigation

If you want the navigation bar to always be on top, you can use z-index:

1.navbar {
2  position: fixed;
3  z-index: 1000;
4}

Modal Windows and Tooltips

For modal windows and tooltips, z-index allows displaying them above the rest of the page:

1.tooltip {
2  position: absolute;
3  z-index: 100;
4}

Potential Problems

Managing z-index values can be complicated in large projects. Setting too many different values can lead to conflicts and difficulties in maintaining the code.