Q1. What is z-index?

z-index controls the stacking order of overlapping elements. Higher values appear on top.

Q2. When does z-index work?

z-index only works on positioned elements (non-static).

Q3. What is the default z-index value?

Default is auto, which means element doesn't create new stacking context and inherits parent's stacking level.

Q4. What is stacking context?

Stacking context is a group of elements with same parent. z-index values are compared within same context.

Q5. Can z-index be negative?

Yes, negative values can place elements behind others, but they may go behind parent background.