線が伸び縮みするアニメーション

.border{
	position: relative;
	width:200px;
	height:3px;
	margin:40px auto;
}

.border:before{
	content: '';
	position: absolute;
	left: 50%;
	bottom: 0;
	width: 0;
	border-bottom: solid 3px #000;
	transform: translateX(-50%);
	animation: border_anim 3s ease-out infinite;
}

@keyframes border_anim {
	0%{
		width: 50%;
	}
	50%{
		width: 100%;
	}
	100%{
		width: 50%;
	}
}