demo/vertical-horizontal-center/vertical-horizontal/2-absolute-margin.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>
        .parent {
            position: relative;
            height: 100px;
            border: 1px solid red;
        }
        .son {
            position: absolute;
            top: 50%;
            left: 50%;
            margin-left: -50px;
            margin-top: -5px;
            width: 100px;
            height: 10px;
            background-color: red;
        }
    </style>
</head>
<body>
    <div class="parent">
        <div class="son"></div>
    </div>
</body>
</html>