<!DOCTYPE html>
<html>
<head>
    <style>
        body {
            margin: 0;
        }
        
        #scroller {
            position: relative;
            z-index: 0;
            width: 400px;
            height: 400px;
            margin: 20px;
            overflow: scroll;
            padding: 20px;
            border: 30px solid orange;
            box-sizing: border-box;
            border-radius: 80px 200px;
            background-color: silver;
        }

        .negative {
            position: absolute;
            top: 20px;
            left: 30px;
            z-index: 1;
            padding: 20px;
            background-color: gray;
            transform: translateZ(0);
        }
        
        .contents {
            height: 1000px;
            background: linear-gradient(green, blue);
        }
        
        .masker {
            position: absolute;
            top: 49px;
            left: 49px;
            width: 342px;
            height: 342px;
            box-sizing: border-box;
            border: 3px solid gray;
            border-radius: 49px 170px;
        }
    </style>
    <script>
        window.addEventListener('load', () => {
            scroller.scrollTo(0, 250);
        }, false);
    </script>
</head>
<body>
    <div id="scroller">
        <div class="negative"></div>
        <div class="contents"></div>
    </div>
    <div class="masker"></div>
</body>
</html>