first-letter-contributed-to-ancestor.html

<!DOCTYPE html>
<link rel="help" href="https://bugs.chromium.org/p/chromium/issues/detail?id=1495811">
<style>
  math {
    display: inline-block;
  }
  math::first-letter {
    background-color: blue;
  }
</style>
<math>
  <mo id="mo">%</mo>
</math>
<script>
  mo.appendChild(document.createTextNode("A"));
  document.body.innerText += "";
</script>

chromium/third_party/blink/web_tests/external/wpt/mathml/crashtests/first-letter-contributed-to-ancestor.html