<!DOCTYPE html>

<html>
<head>
    <style>
        .background {
            height: 100px;
            width: 100px;
            padding: 50px;
            background-image: repeating-linear-gradient(black 0px, black 25px, white 25px, white 50px);
            border: 1px solid black;
        }
        
        #box {
            height: 100px;
            width: 100px;
            -webkit-backdrop-filter: blur(0px);
        }
        
        #box.changed {
            -webkit-backdrop-filter: blur(20px);
        }
    </style>
    <script>
        if (window.testRunner)
            testRunner.waitUntilDone();

        window.addEventListener('load', function() {
            window.setTimeout(function() {
                document.getElementById('box').classList.add('changed');
                window.setTimeout(function() {
                    if (window.testRunner)
                        testRunner.notifyDone();
                }, 0);
            }, 0);
        }, false);
    </script>
</head>
<body>
<div class="background">
    <div id="box">
    </div>
</div>
</body>
</html>