<!DOCTYPE html>
<link rel="help" href="https://www.w3.org/TR/CSS2/visuren.html#propdef-z-index">
<link rel="help" href="https://www.w3.org/TR/compositing-1/">
<link rel="author" title="Chris Harrelson" href="mailto:chrishtr@chromium.org">

<div style="position: relative; will-change: transform; z-index: -1"></div>
<div id="target"
     style="width: 50px; height: 50px; background: blue; opacity: 0.99"></div>