Unstacking CSS Stacking Contexts

Have you ever set z-index: 99999 on an element in your CSS, and it doesn’t come out on top of other elements? A value that large should easily place that element visually on top of anything else, assuming all the different elements are set at either a lower value or not set at all.
A webpage is usually represented in a two-dimensional space; however, by applying specific CSS properties, an imaginary z-axis plane is introduced to convey depth. This plane is perpendicular to the screen, and