On October 10, 1994, Hkon Wium Lie introduced CSS for the first time. Lie was working at CERN at the time alongside Tim Berners-Lee.
The z-order of a positioned element and its descendants or flex items is determined by the z-index CSS attribute. Those with a smaller z-index are covered by elements having a bigger z-index.
A margin is the space around an element's border in CSS, whereas padding is the space between the element's border and its content. To put it another way, the margin and padding properties manage the space outside and inside an element, respectively.