<h1 class="cp_h1title">見出しテキスト</h1>

.cp_h1title {
	position: relative;
	padding: .3em;
	border-bottom: 1px solid #7986CB;
}
.cp_h1title::after {
	position: absolute;
	bottom: -3px;
	left: 0;
	z-index: 2;
	content: '';
	width: 15%;
	height: 5px;
	background-color: #303F9F;
}