The z-index property sets the stacking order of elements. Elements with higher stacking order are always in front of elements with lower stacking order.
The attribute value of the attribute can have negative values;
z-index : auto | number
auto: Default value.
number: A unitless integer value, which can be negative. 100 max.
Elements with larger z-index values will be stacked on top of elements with smaller z-index values. For positioned objects that do not specify this property, objects with a positive z-index value will be above it, and objects with a negative z-index value will be below it.
The z-index attribute applies to positioning elements (objects whose position attribute value is relative or absolute or fixed), and is used to determine the stacking order of positioning elements in the direction perpendicular to the display screen (called the Z axis).