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chromium/third_party/blink/web_tests/paint/subpixel/subpixel-composited-iframe.html 

<!DOCTYPE html>
<iframe id="iframe"
  style="position: absolute; top: 5.5px; left: 5.5px; border: none; width: 200px; height: 200px"
  srcdoc="
      <div id='target' style='width: 100px; height: 100px; background: black; will-change: transform'>
      </div>
  ">
</iframe>