Two positioned siblings overlap. Box A has z-index: 9999 and Box B has z-index: 1, but A renders behind B. Which single explanation is consistent with this?

<div class="parent-a"> <!-- opacity: 0.99; --> <div class="a">A</div> <!-- position: relative; z-index: 9999 --> </div> <div class="parent-b"> <!-- no special props --> <div class="b">B</div> <!-- position: relative; z-index: 1 --> </div>