<p>線の幅が「3px」、間隔が「5px」</p>
<div class="example"></div>

body {
  padding: 10px 5%;
}
.example {
  height: 1px;
	position: relative;
}
.example::before {
	content: "";
	background-image: linear-gradient(to right, #000, #000 3px, transparent 3px, transparent 8px);
	background-size: 8px 1px;
	background-repeat: repeat-x;
	position: absolute;
	top: 0;
	bottom: 0;
	left: 0;
	right: 0;
}