<script>
if (window.internals)
    internals.settings.setConstantPropertiesEnabled(true);
</script>
<style>
div {
    position: absolute;
    width: 100px;
    height: 100px;
    background-color: green;

    --valid-fallback: 100px;

    top: env(some-unknown-name, var(--valid-fallback));
    left: env(--some-invalid-name, var(--valid-fallback));
}
</style>
<body>
<div></div>
<p>The box should be at 100, 100.</p>

