screenshots.gif

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width: 100px;
            height: 100px;
            background-color: pink;
        }
        .new{

            position: absolute;
            width: 200px;
            height: 200px;
            left: 200px;
            top:200px;
        }
    </style>
</head>
<body>
    <input type="button" value="按钮" id="btn">
    <div id="box" </div>
        <script>
function my$(id){return document.getElementById(id)};

            //获取元素
            var btn = my$("btn");
            var box = my$("box");
            //添加事件
            btn.onclick = function (){
                //修改类名
               // box.className = "new";
                //直接修改样式属性
                box.style.position = "absolute";
                box.style.width = "200px";
                box.style.height = "200px";
                box.style.left = "200px";
                box.style.top = "200px";
            }

        </script>
    </body>
</html>