chromium/third_party/blink/web_tests/fast/block/positioning/move-with-auto-width.html

<div style="width: 60px; height: 40px; position: relative;">
    <div style="width: 20px; height: 40px; background-color: red; margin-left: 20px;"></div>
    <div id="target" style="position: absolute; top: 0; left: 0; font-family: Ahem; font-size: 20px; color: green;">
        X X
    </div>
</div>
<script>
    document.body.offsetTop;
    document.getElementById("target").style.left = "20px";
</script>