<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>已知宽高实现盒子水平垂直居中</title>
<style type="text/css">
.box {
border: 1px solid #00ee00;
height: 300px;
position：relative;
}
.box1 {
position:absolute;
top:50%;
left:50%;
margin:-100px 0 0 -100px;
width: 200px;
height: 200px;
border: 1px solid red;
}
</style>
</head>
<body>
<div class="box">
<div class="box1"></div>
</div>
</body>
</html>
position：relative;这个代码有误，此冒号非英文输入法的冒号。