margin-left:auto;
    margin-right:auto;

    1. <!DOCTYPE html>
    2. <html lang="en">
    4. <head>
    5.     <meta charset="UTF-8">
    6.     <meta name="viewport" content="width=device-width, initial-scale=1.0">
    7.     <title>Document</title>
    8.     <style>
    9.         div {
    10.             margin-left: auto;
    11.             margin-right: auto;
    12.             width: 100px;
    13.             height: 100px;
    14.             background-color: red;
    15.         }
    16.     </style>
    17. </head>
    18. <body>
    19.     <div>
    20.     </div>
    21. </body>
    22. </html>