CSS z-index Property
In CSS, the z-index property governs the vertical stacking order of elements that overlap. In other words, which one appears to be physically closer to you. The z-index only affects elements with non-static position values (the default).
Elements can overlap for a variety of reasons, such as relative positioning, which has nudged one element over another. A negative margin has pushed one element ahead of another. Elements that are perfectly positioned overlap.
|auto||Sets the stack order to the same as its parents. This is the default setting.|
|number||Sets the element's stack order. Negative numbers are permitted.|
|initial||This property is set to its default value.|
|inherit||This property is inherited from its parent element.|
|Property||Chrome||Firefox||Safari||Edge / IE||Opera|