动画背景没有固定

嘿，伙计们，我正在为我的网站开发一个新的背景。我试着把它动画化一点。我目前在这里工作：http://wow-x.com/back/

正如您所看到的，当您向下滚动页面时，名为"background"的div不会保持"固定"。。。我似乎无法使它保持固定。

我的CSS如下：

body {
    background-color: #0a0a0a;
    color: #a2a2a2;
    font-family: Arial, Helvetica, sans-serif;
    font-size: 12px;
    line-height: 1.4;
    margin: 0;
    padding: 0;
    background-image: url(http://wow-x.com/board/images/blackevo4-pure/background.jpg);
    background-repeat: no-repeat;
    background-position: center top;
    background-attachment:fixed;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
}
#background {
    background: url(http://wow-x.com/board/images/blackevo4-pure/back1.jpg);
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    min-width: 100%;
    min-height: 100%;
    opacity:0.1;
    z-index:-1;
}
#midground {
    background: url(http://wow-x.com/board/images/blackevo4-pure/midground.png) repeat 20% 20%;
    background-attachment:fixed;
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    z-index:-2;
    height:100%;
    width:100%;
}
#foreground {
    background: url(http://wow-x.com/board/images/blackevo4-pure/foreground.png) repeat 90% 110%;
    background-attachment:fixed;
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    z-index:-3;
    height:100%;
    width:100%;
}