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chromium/third_party/blink/web_tests/scrollbars/resources/reconstruction-iframe-src.html 

<!DOCTYPE html>
<style>
::-webkit-scrollbar {
    width: 50px;
    height: 20px;
}
::-webkit-scrollbar-thumb {
    background: green;
}
div {
    width: 400px;
    height: 400px;
}
</style>
<div style="width:400px;height:400px;"></div>