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chromium/third_party/blink/web_tests/external/wpt/css/css-scrollbars/support/viewport-scrollbar-frame.html 

<!DOCTYPE html>
<meta charset="UTF-8">
<style>
html {
  scrollbar-color: yellow blue;
}
html, body {
  margin: 0;
  padding: 0;
}
#inner {
  width: 400px;
  height: 400px;
}
</style>
<div id="inner"></div>