18. How do you change the display order of elements using CSS?

You can use the z-index property to control the display order of elements with positioning other than static. The z-index value specifies the stacking order of elements along the Z-axis (depth), with elements having higher z-index values displayed on top.

/* Examples */
.element1 {
    position: absolute;
    z-index: 1; /* Element will be below element2 */
}

.element2 {
    position: absolute;
    z-index: 2; /* Element will be above element1 */
}