mozilla-release/layout/style/crashtests/1383981-3.html

<!doctype html>
<style>
div { padding: 1px; }
.test div { padding: 2px; }
.test div div { padding: 3px; }
.test div div div { background: orange; }
.test div div div div { background: white; }
.test div div div div div { background: red; }
</style>
<body>
<script>
let root = document.createElement('div');
let p = root;
for (let i = 0; i < 1000; ++i) {
  p = p.appendChild(document.createElement('div'));
  p.appendChild(document.createTextNode(i));
}
document.body.appendChild(root);

// Flush styles.
document.body.offsetTop;

// Add 20 more top-level siblings to ensure that the style traversal goes
// parallel before the deep tree is processed.
//
// Note that we need to make these children of the <html> element, not the
// <body> element, because invalidations get processed by the parent when
// enqueuing children, so the _parent_ needs to be at a level in the DOM
// with enough dirty siblings to trigger a switch to parallel mode.
for (let i = 0; i < 20; ++i) {
  document.documentElement.appendChild(document.createElement('div'));
}

root.className = 'test';
</script>
</body>