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chromium/third_party/blink/web_tests/fast/ruby/rt-bounding-rect.html 

<!DOCTYPE html>
<script src="../../resources/ahem.js"></script>
<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>
<style>
body {
  font: 20px/1 Ahem;
}
rt {
  /* background is important to generated a PhysicalBoxFragment. */
  background: orange;
}
</style>
<p>before <ruby><span>base</span><rt>annotati</ruby></p>
<script>
function getRect(query) {
  return document.querySelector(query).getBoundingClientRect();
}

test(() => {
  assert_equals(getRect('span').x, getRect('rt').x);
}, 'Inline offset of a <rt> is same as the base text if text widths are same');
</script>