CSS3动画 transition 过渡，宽度由中间向两边发生过渡

.tapered_line{
    width: 36px;
    height: 4px;
    background: #1e88e5;
    transition: width 3s;
    margin: 10px auto 0;
}
.main4 ul li:hover .tapered_line{
    width: 220px;
    margin:10px auto 0;
}