为啥蓝色的背景出不来？

.topList ul li em{
    display:block;
    width:20px;
    height:16px;
    text-align:center;
    font-style:normal;
    background:url(http://img.mukewang.com/53cf0fa20001d3dc00200032.jpg) no-repeat;
    background-position:0 -16px;
    float:left;
}


.top em{
        background-position:0 0;
        color:#fff;
}