背景固定的效果fixed - background-attachment:fixed

- background-attachment:fixed

兼容写法：

.active_main::before {   content: ' ';   position: fixed;   z-index: -1;   top: 0;   right: 0;   bottom: 0;   left: 0;   background: url(../img/qmsd_bg.jpg) no-repeat 0 top #f2f8fa;   background-size: 100% auto; }