mask 半透明蒙版

var MAX_Z_INDEX=100;    //记录页面中的最大zindex值

$("#xx").show().css({position:'absolute',top:0,left:0,zIndex:++MAX_Z_INDEX});

#mask{
    position:absolute;
    top:0;
    left:0;
    800px;
    height:600px;
    background:#000;
    filter:alpha(opacity=75);
    -ms-filter:"alpha(opacity=75)";
    opacity:.75;
    z-index:99;
}

<div id="mask" style="display:none;"></div>

$("#mask").show();