vscode编辑
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .shadow{
            position: fixed;
            top: 0;
            bottom: 0;
            left: 0;
            right: 0;
            background-color:rgba(0,0,0,0.5);
            z-index: 80;
        }
        .mode{
            position: fixed;
            height: 400px;
            width: 400px;
            background-color: white;
            z-index: 100; /*数值越大，越在上层显示*/
            left: 50%; /*按照窗口宽度的50%来移动*/
            top: 50%; /*按照窗口高度的50%来移动*/
            margin-top: -200px; 
            margin-left: -200px;/*正的相当于距离左边200px，负号相当于右边了 */
            
        }
    </style>
</head>
<body>
    <div>
        <h1>
            我在测试
        </h1>
    </div>
    
    <div class="mode"></div>
    <div class="shadow"></div>
</body>
</html>