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chromium/third_party/blink/web_tests/external/wpt/css/css-scrollbars/scrollbar-color-012-ref.html 

<!doctype html>
<html>
<style>
  .container {
    overflow: auto;
    height: 200px;
    width: 200px;
    background-color: red;
  }
</style>
<div class="container">
  <div class="content"></div>
</div>