Underline transition from middle out - ref: http://bradsknutson.com/blog/css-sliding-underline/

style.css

.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;
}