<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>