css不显示二级下拉菜单

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>CSS Dropdown Menu</title>
    <style>
        .menu {
            list-style: none;
            padding: 0;
            margin: 0;
        }
        .menu-item {
            display: inline-block;
            position: relative;
        }
        .sub-menu {
            display: none;
            position: absolute;
            top: 100%;
            left: 0;
            background-color: #fff;
            border: 1px solid #ccc;
            padding: 0;
            margin: 0;
        }
        .menu-item:hover .sub-menu {
            display: block;
        }
    </style>
</head>
<body>
    <ul class="menu">
        <li class="menu-item">
            主菜单项
            <ul class="sub-menu">
                <li>子菜单项1</li>
                <li>子菜单项2</li>
            </ul>
        </li>
    </ul>
</body>
</html>