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chromium/third_party/blink/web_tests/fast/css/nth-child-of-pseudo-element-assert.html 

<!DOCTYPE html>
<style>
div::before {
    position: relative;
    overflow: scroll;
    display: block;
    width: 100px;
    height: 100px;
    content: "."
}

:nth-child(13)::-webkit-scrollbar {
    color: pink;
}
</style>
<script>
if (window.testRunner)
    testRunner.dumpAsText();
</script>
<p>This test should not trigger any ASSERTs.</p>
<div></div>