The Z-Index and Layering
On the last page, we learned how to place elements “wherever” we want. We even learned how to place elements over other elements, thereby, blocking those elements.
The z-index property allows developers to specify which elements should be placed on top of other elements. The z-index property takes an integer number as its value (i.e. -1000 to 1000). Those elements with higher z-index values will be drawn on top elements with lower values.