<iframe id=frame srcdoc="
<style>
@media (min-width:200px) {
    @font-face {
        font-family: family1;
        src: local(times);
    }
    body { background-color: red }
}

@media (max-width:200px) {
    @font-face {
        font-family: family1;
        src: local(courier);
    }
    body { background-color: green }
}
body { font-family:family1 }
</style>
Test frame
"></iframe>
<script>
frame.onload = () => {
    frame.contentDocument.offsetWidth;
    frame.width = 100;
}
</script>