<head>
<meta charset="utf-8" />
<title></title>
<!--内部样式等同于cass文件-->
<style>
#parent
{
width: 200px;
height: 200px;
background-color: burlywood;
position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%,-50%);
}
#child
{
/*方法一:绝对定位position:absolute+left根据整个页面定位position: relative:相对定位根据容器 */
/*方法二:下面四行代码全上表示相对定位/*/
1: position: relative;
2: top: 50%;
3:left: 50%;
4: transform: translate(-50%,-50%);
width: 100px;
height: 100px;
background-color: cadetblue;
/*透明度*/
opacity:0.5 ;
}
</style>
</head>
<body>
<div id="parent">
<div id="child"></div>
</div>
</body>