搜集翻译了一些下划线的动态效果，感觉可能什么时候会用，这里贴一下~请使用现代浏览器查看效果。

主要就是使用了CSS的:after伪类，再加上一些transition动画效果，设置设置宽度高度，左右距离好像就完事儿了……

下划线从上到下出现

鼠标放上去，你会发现下划线“生长”出来了，不过页面所有的内容也往下走了3像素，这样视觉效果不是很好，你可以将它设置一下绝对定位（就好像下面的那个例子）或者用一个固定高度的容器包裹一下它。

.sliding-u-t-b {
    display: inline-block;
}
.sliding-u-t-b:after {
    content: '';
    display: block;
    height: 0;
    width: 100%;
    background: transparent;
    transition: height .5s ease, background-color .5s ease;
}
.sliding-u-t-b:hover:after {
    height: 3px;
    background: blue;
}

下划线从下到上出现

这个就是使用绝对定位的效果。

.sliding-u-b-t {
	display: inline-block;
	position: relative;
}
.sliding-u-b-t:after {
	content: '';
	position: absolute;
	bottom: 0;
	left: 0;
	height: 0px;
	width: 100%;
	background: transparent;
	-webkit-transition: all ease 0.5s;
	-moz-transition: all ease 0.5s;
	transition: all ease 0.5s;
}
.sliding-u-b-t:hover:after {
	height: 3px;
	background: blue;
}

下划线从左到右出现

.sliding-u-l-r {
display: inline-block;
}
.sliding-u-l-r:after {
content: '';
display: block;
height: 3px;
width: 0;
background: transparent;
transition: width .5s ease, background-color .5s ease;
}
.sliding-u-l-r:hover:after {
width: 100%;
background: blue;
}

下划线从右到左出现

.sliding-u-r-l {
    display: inline-block;
    position: relative;
    padding-bottom: 3px;
}
.sliding-u-r-l:after {
    content: '';
    display: block;
    position: absolute;
    right: 0;
    bottom: 0;
    height: 3px;
    width: 0;
    background: transparent;
    transition: width .5s ease, background-color .5s ease;
}
.sliding-u-r-l:hover:after {
    width: 100%;
    background: blue;
}

下划线两边伸展

.sliding-middle-out {
	display: inline-block;
	position: relative;
	padding-bottom: 3px;
}
.sliding-middle-out:after {
	content: '';
	display: block;
	margin: auto;
	height: 3px;
	width: 0px;
	background: transparent;
	transition: width .5s ease, background-color .5s ease;
}
.sliding-middle-out:hover:after {
	width: 100%;
	background: blue;
}
.sliding-middle-out {
	display: inline-block;
	position: relative;
	padding-bottom: 3px;
}
.sliding-middle-out:after {
	content: '';
	display: block;
	margin: auto;
	height: 3px;
	width: 0px;
	background: transparent;
	transition: width .5s ease, background-color .5s ease;
}
.sliding-middle-out:hover:after {
	width: 100%;
	background: blue;
}

下划线滑进滑出

.sliding-u-l-r-l {
    display: inline-block;
    position: relative;
    padding-bottom: 3px;
}
.sliding-u-l-r-l:before {
    content: '';
    display: block;
    position: absolute;
    left: 0;
    bottom: 0;
    height: 3px;
    width: 0;
    transition: width 0s ease, background .5s ease;
}
.sliding-u-l-r-l:after {
    content: '';
    display: block;
    position: absolute;
    right: 0;
    bottom: 0;
    height: 3px;
    width: 0;
    background: blue;
    transition: width .5s ease;
}
.sliding-u-l-r-l:hover:before {
    width: 100%;
    background: blue;
    transition: width .5s ease;
}
.sliding-u-l-r-l:hover:after {
    width: 100%;
    background: transparent;
    transition: all 0s ease;
}

下划线反向滑进滑出

.sliding-u-l-r-l-inverse {
    display: inline-block;
    position: relative;
    padding-bottom: 3px;
}
.sliding-u-l-r-l-inverse:before {
    content: '';
    display: block;
    position: absolute;
    left: 0;
    bottom: 0;
    height: 3px;
    width: 100%;
    transition: width 0s ease;
}
.sliding-u-l-r-l-inverse:after {
    content: '';
    display: block;
    position: absolute;
    right: 0;
    bottom: 0;
    height: 3px;
    width: 100%;
    background: blue;
    transition: width .5s ease;
}
.sliding-u-l-r-l-inverse:hover:before {
    width: 0%;
    background: blue;
    transition: width .5s ease;
}
.sliding-u-l-r-l-inverse:hover:after {
    width: 0%;
    background: transparent;
    transition: width 0s ease;
}