html css样式绘制向下或者向上箭头

.ico_up{
    width:0;
    height:0;
    border:4px solid transparent ;
    border-bottom-color:#B0B0B0;
}
.ico_down{
    float: top;
    width:0;
    height:0;
    border:4px solid transparent ;
    border-top-color:#B0B0B0;

}