demo/vertical-horizontal-center/horizontal/3-margin-auto-fit-content.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .margin-auto {
            margin: 0 auto;
        }
        .red {
            background-color: red;
            width: 300px;
            height: 20px;
        }
        .float-left {
            float: left;
        }
        .fit-content {
            width: -moz-fit-content;
            width: -webkit-fit-content;
            width: fit-content;
        }
    </style>
</head>
<body>
    <div class="margin-auto red">
    </div>
    <br>
    <div class="margin-auto">
        <div class="red float-left">margin-auto 元素没有加 fit-content </div>
    </div>
    <br>
    <div class="margin-auto fit-content">
        <div class="red float-left">margin-auto 元素加了fit-content</div>
    </div>
    
</body>
</html>