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chromium/third_party/blink/web_tests/compositing/scrollbars/composited-custom-scrollbar.html 

<!DOCTYPE html>
<style>
::-webkit-scrollbar {
  width: 20px;
  height: 20px;
  background: blue;
}
::-webkit-scrollbar-thumb {
  background: green;
}
::-webkit-scrollbar-corner {
  background: yellow;
}
</style>
<div style="width: 100px; height: 100px; overflow:scroll; will-change: transform; resize: both">
  <div style="width: 320px; height: 160px"></div>
</div>