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chromium/third_party/blink/web_tests/scrollbars/scrollbar-corner-colors.html 

<!DOCTYPE html>
<div style="width:10000px;height:10000px">
  <div style="height:400px;width:400px;overflow:scroll;border:solid 2px black;">
  <p style="width:450px;height:450px;">Create a scrollable area including scroll corner</p>
  </div>
  <p>Create large enough content to cause a scroll corner in the root viewport</p>
</div>