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chromium/third_party/blink/web_tests/external/wpt/html/semantics/popovers/popover-anchor-inset-rule-display-ref.html 

<!DOCTYPE html>
<meta charset="utf-8">
<link rel="stylesheet" href="resources/popover-styles.css">

<div class="fake-popover" anchor></div>

<style>
  .fake-popover {
    background: green;
    width: 100px;
    height: 100px;
  }
  body { margin: 0; }
</style>