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chromium/third_party/blink/web_tests/external/wpt/css/css-transitions/transition-base-response-002.html 

<!DOCTYPE html>
<title>Test that rem units are responsive to transitioning font-size on root</title>
<link rel="help" href="https://drafts.csswg.org/css-transitions/">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  :root {
    font-size: 20px;
  }
  :root.change {
    transition: font-size steps(2, start) 10s;
    font-size: 30px;
  }

  #target1 {
    width: 1rem;
  }
</style>
<div id="target1"></div>
<output id=output></output>
<script>
test(() => {
  let unused = getComputedStyle(document.documentElement).getPropertyValue('font-size');
  document.documentElement.className = 'change';
  assert_equals(getComputedStyle(target1).getPropertyValue('width'), '25px');
}, 'Transitioning font-size on root affects rem units');
</script>