What is "z-index"

z-index is a CSS property that sets the stack order of specific elements. An element with greater stack order is always in front of another element with lower stack order.

p { position: relative; z-index: -1; }

Negative stack orders can also be used in the same manner. A negative value will appear behind a positive one. z-index only works on elements that have a position value (e.g. position: relative;). Like all other CSS properties, it can be set with JavaScript as well with the following syntax:

object.style.zIndex="1"