Who's in Charge Here?

z-index is depth (layers). But it doesn't work as simply as it seems.

1. Stacking Context

This is a " group of layers " . Imagine you live in a two-story house:

• If you're higher than your neighbor in the room (z-index property), you're " above " .
• But if you're on the 1st floor and neighbor is on the 2nd, no matter how high your stool (z-index) is — the neighbor on the 2nd floor will ALWAYS be above you to an outside observer.

2. What Creates a New Context?

• position: absolute/relative + z-index (not auto).
• opacity less than 1.
• transform (any, even scale(1)).
• filter, perspective.