<!DOCTYPE html>
<html>
<head>
    <style>
        .container {
            width: 400px;
            border: 20px solid blue;
            background-color: rgba(0, 128, 0, 0.2);
            filter: url(#filter);
            -webkit-clip-path: inset(30px 320px 30px 30px); /* work around webkit.org/b/189896 */
        }
    
        #box {
            width: 100px;
            height: 100px;
            background-color: green;
            margin-left: 300px;
        }
        </style>
</head>
<body>
    <svg style="display: none">
        <filter id="filter">
            <femorphology operator="dilate" radius="10" in="SourceAlpha" result="morphology"></femorphology>
            <femerge result="merge">
                <femergenode in="morphology"></femergenode>
                <femergenode in="SourceGraphic"></femergenode>
            </femerge>
        </filter>
    </svg>

    <div class="container">
        <div id="box"></div>
    </div>
</body>
</html>