Which statement is true about z-index across stacking contexts?

• A. Z-index values are compared across all stacking contexts.
• B. Z-index only affects stacking order within the same stacking context.
• C. Z-index creates a new stacking context.
• D. Z-index has no effect on stacking order.

Answer: (B)

Z-index orders elements inside a stacking context, which acts like its own mini layering system. Inside one stacking context, elements with z-index values are compared to determine which sits on top. But when there are multiple stacking contexts—each created by different parts of the page—the z-index values of elements in one context don’t cross into another. The entire stacking context is treated as a single unit relative to its sibling contexts, and the internal z-index values can’t change how those contexts are ordered with respect to each other.

That’s why the statement is true: z-index affects stacking only within the same stacking context. If two elements each establish their own contexts, you’ll stack those whole contexts in the order determined by their parent’s rules, and the internal z-index values stay inside their respective contexts.