移动端背景固定
background-attachment:fixed;在pc端有效，移动端无效，解决如下：
body:before {
content: ' ';
position: fixed;
z-index: -1;
top: 0;
right: 0;
bottom: 0;
left: 0;
background: url(...) center 0 no-repeat;
background-size: cover;
}