1.postiton 元素已知宽度
<style> /* 父相对 子绝对 子上下 margin l t -一半PX */ #box{ background-color: lightblue;width: 300px;height:300px; position: relative; } #content{ position: absolute;background-color: tomato; height: 100px;width: 100px; left: 50%;top: 50%;margin: -50px 0 0 -50px; } </style> </head> <body> <div id="box"> <div id="content"> </div> </div> </body>