<iframe id=frame srcdoc="
<style>
body { background-color: red }

@media (min-width:400px) {
    body { background-color: green }

    @keyframes fade {
        0% { opacity: 0; }
        100% { opacity: 1; }
    }
}
</style>
Test frame
"></iframe>
<script>
frame.onload = () => {
    frame.width = 100;
    frame.contentDocument.offsetWidth;
    frame.width = 500;
}
</script>