A parent has `opacity: 0.99` and a child inside it has `z-index: 2147483647` (the max). A sibling of the parent has `z-index: 5`. The child still cannot appear above that sibling. What property of stacking contexts explains this?
<div style="opacity:.99">
<span style="position:relative; z-index:2147483647">child</span>
</div>
<div style="position:relative; z-index:5">sibling</div>