Stacking: z-index and What Sits on Top

When boxes overlap, the browser stacks them like papers on a desk, and z-index, positioning and stacking contexts decide which one ends up on top, including why z-index 9999 sometimes still loses.

Updated

Cheat sheet: Stacking: z-index and What Sits on Top
Category
CSS
Level
Journeyman
Tags
CSS, Stacking context, Stacking order, Z-index