<!DOCTYPE html>
<style>
    body {
        height: 1000px;
        padding-top: 300px;
    }
    body > div {
        display: inline-block;
        width: 100px;
        height: 100px;
        background: red;
    }
    body > div > div {
        position: fixed;
        width: 100%;
        height: 100%;
        top: 0;
        background: green;
    }
</style>
You should see three green boxes.<br>

<div style="transform:scale(1)"><div></div></div>
<div style="perspective:0"><div></div></div>
<div style="transform-style:preserve-3d"><div></div></div>

<script>
    // Scroll on load to test fixed positioning.
    scrollTo(0, 300);
</script>