居中一个浮动元素

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        /*//居中div*/
        .div1{
            width:300px;
            height:300px;
            background-color: red;
            margin:0 auto;
        }

        /*居中一个浮动元素*/
        .div2{
            width:300px;
            height:300px;
            background-color: red;
            float: left;
            position: relative;
            left:50%;
            margin-left: -150px;
        }
    </style>
</head>
<body>
<div class="div1"></div>
<div class="div2"></div>
</body>
</html>