body:before {
content: ‘ ‘;
position: fixed;
z-index: -1;
top: 0;
right: 0;
bottom: 0;
left: 0;
background: url(bj3.jpg) center 0 no-repeat;
background-size: cover;
}
原文：http://www.cnblogs.com/zpweb/p/6015046.html