登录页设计（登录盒子居中）
.login_container内有一个login_box
设置login_box居中
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>登录页设计（窗体居中设计）</title>
<style>
html,body,#app{
height: 100%;
margin:0;
}
.login_container{
background-color: #2b4b6b;
height: 100%;
}
.login_box{
width: 450px;
height: 300px;
background-color: #fff;
border-radius: 3px;
position: absolute;
left: 50%;
top: 50%;
transform: translate(-50%,-50%);
}
</style>
</head>
<body>
<div id="app">
<div class="login_container">
<div class="login_box">123</div>
</div>
</div>
</body>
</html>
