#header_nav {
    height: 91px;
    background-color: transparent;
    text-align: center;
    position: fixed;
    width: 100%;
}

#nav_content {
    margin: 27px auto auto auto;
    padding-left: 257px;
    height: 46px;
    width: auto;
    position: relative;
    display: inline-block;
}

img {
    height: 46px;
    width: 128px;
    position: absolute;
    left: 0;
}

#title_span {
    height: 14px;
    margin-top: 11px;
    width: 109px;
    font: 16px SourceHanSansCN-ExtraLight bold;
    color: white;
    position: absolute;
    left: 130px;
}

ul {
    width: auto;
    margin-left: 20px;
}

li {
    display: inline-block;
    height: 18px;
    padding: 3px 0;
    margin-right: 37px;
    margin-top: 11px;
}

li > a {
    color: white;
    text-decoration: none;
}
.dowebok a {
    font-size: 22px;
    padding-right: 50px;
    text-decoration: none;
    color:#000000;
}
.dowebok a:hover {
    text-decoration: none;
    color: red;
}
/* 移入 */
#underline {
    position: relative;
}
#underline::before,
#underline:after {

    content: "";
    /*单引号双引号都可以，但必须是英文*/
    width: 0;
    /*默认0 */
    height: 3px;
    /*下划线高度*/

    background: #fff;
    /*下划线颜色*/

    position: absolute;

    top:200%;

    left: 50%;

    transition: all .4s;
    /*css动画效果，0.8秒完成*/

}

#underline:hover:before {
    /*动画效果是从中间向左延伸至50%的宽度*/

    left: 0%;

    width: 50%;

}

#underline:hover:after {
    /*动画效果是从中间向右延伸至50%的宽度*/

    left: 50%;
    /*这句多余，主要是为了对照*/

    width: 50%;

}