以下1,2两个关键元素
body{
width:100%;
height:100%;
position:relative; //1
}
.login-form {
width: 100%;
background:url(./img/loginbg.png) no-repeat;
height:100%;
background-size: 100% 100%;
position: absolute; //2
background-position:center;
z-index:4;
}