background兼容IE9以下版本

.box {
    width:100%;
    height:80%;    
    background: url('img/nav_bg.png') no-repeat;
    background-attachment: fixed;
    filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(src='img/nav_bg.png', sizingMethod='scale');
    -ms-filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(src='img/nav_bg.png', sizingMethod='scale');
    background-size: cover;
    -moz-background-size: cover;
    -webkit-background-size: cover;
    font-family: "微软雅黑";
   }