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chromium/third_party/blink/web_tests/virtual/text-antialias/whitespace/attach-whitespace-after-before-pseudo.html 

<!DOCTYPE html>
<style>
  #container::before { content: "X" }
</style>
<p>There should be a space between the Xs below.</p>
<div id="container">
  <span>X</span>
</div>