reattach-before-pseudo-slot-fallback-whitespace.html

<!DOCTYPE html>
<style>
  #host::before { content: "A" }
  #host.block::before { display: block }
</style>
<p>You should see a space between A and B below.</p>
<div id="host" class="block"></div>
<script>
  var root = host.attachShadow({mode:"open"});
  root.innerHTML = '<slot name="fallback"> </slot>B</div>';
  host.offsetTop;
  host.className = "";
</script>

chromium/third_party/blink/web_tests/virtual/text-antialias/whitespace/reattach-before-pseudo-slot-fallback-whitespace.html