CSS 2.1 - position absolute and z index
example 1 - stacking of elements in the normal flow
example 2 - stacking of positioned elements without z-index
example 3 - stacking of elements with position absolute and z-index
example 4 - elements with z-index establish their own stacking context
https://vanseodesign.com/css/css-stac...
https://philipwalton.com/articles/wha...