Which statement best describes a stacking context and how z-index is evaluated?

• A. Z-index is global across all elements regardless of stacking contexts.
• B. A stacking context is created by position, opacity, and transform; z-index comparisons happen only within the same stacking context.
• C. Z-index affects elements regardless of stacking contexts.
• D. A stacking context is created by font-size changes.

Answer: (D)

Z-index is meaningful only within the stacking context that an element belongs to. A stacking context is created by several triggers, including an element that is positioned (not static) with a z-index value other than auto, or an element that has opacity less than 1, or an element with a CSS transform applied, among other properties. Within a single stacking context, z-index values are compared to determine the painting order. However, z-index does not cross stacking context boundaries; elements in different stacking contexts don’t interact with each other’s z-index. Because font-size changes do not create a stacking context, that option is not a valid way to establish one. In short, z-index is evaluated only within the same stacking context, and stacking contexts themselves are created by properties like position with a non-auto z-index, opacity less than 1, or transform.