<script>
if (window.internals)
    internals.settings.setConstantPropertiesEnabled(true);
</script>
<style>
html {
    --safe-area-inset-bottom: 50px;
    safe-area-inset-left: 50px;
}

#a {
    position: absolute;
    --safe-area-inset-right: 100px;
    safe-area-inset-top: 100px;
    left: env(safe-area-inset-right);
    top: env(safe-area-inset-top);
    width: 100px;
    height: 100px;
    background-color: green;
}

#b {
    position: absolute;
    --safe-area-inset-bottom: 200px;
    safe-area-inset-left: 200px;
    left: env(safe-area-inset-bottom);
    top: env(safe-area-inset-left);
    width: 100px;
    height: 100px;
    background-color: blue;
}
</style>
<body>
<div id="a"></div>
<div id="b"></div>
<p>The boxes should both be at 0, 0.</p>