<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title></title>
        <style>
            * {
                margin: 0;
            }

            .a {
                width: 300px;
                height: 300px;
                background-color: aquamarine;
                position: absolute;
                left: calc(50% - 150px);
                top: calc(50% - 150px);

            }

            .b {
                width: 200px;
                height: 200px;
                background-color: pink;
                position: absolute;
                left: 50%;
                top: 50%;
                margin-top: -100px;
                margin-left: -100px;
            }

            .c {
                width: 100px;
                height: 100px;
                background-color: orange;
                margin: auto;
                position: absolute;
                top: 0;
                right: 0;
                bottom: 0;
                left: 0;
            }
        </style>
    </head>
    <body>
        <div class="a">
            方法1
        </div>
        <div class="b">
            方法2
        </div>、
        <div class="c">
            方法3
        </div>
    </body>
</html>

固定定位同样适用！！！