<head>
<style>
@font-face {
font-family: 'custom';
src: url(firstInvalidURL), url(secondInvalidURL), local('Ahem');
}
</style>
</head>
<body>
<div style="font-family: 'custom'; font-size: 20px; color: green; background-color: red; width: 5em;">
_fail
fail_
_fail
fail_
_fail
</div>
<script>
document.body.offsetTop;
</script>
</body>