<!DOCTYPE html>
<html>
<head>
    <title>The green box should not disappear</title>
    <style>
        .box {
            width: 400px;
            background-color: green;
            height: 0px;
            opacity: 1;
        }
        
        .content {
            height: 200px;
            width: 200px;
            background-color: green;
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="content"></div>
    </div>
</body>
</html>