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chromium/third_party/blink/web_tests/external/wpt/css/css-ui/negative-outline-offset.html 

<!DOCTYPE html>
<link rel="help" href="https://www.w3.org/TR/css-ui-4/#outline-offset">
<link rel="match" href="negative-outline-offset-ref.html">
<style>
  div {
    outline: 1px solid red;
    outline-offset: -10em;

    border: 2px solid black;
    padding: 5px;
    font-size: 16px;
  }
</style>

<p>PASS if there is a thin red line in the middle of the box.</p>
<div></div>