SOURCE

<div>
	<dl>
		<dt>a</dt>
		<dd>alive</dd>
		<dd>about</dd>
		<dt>b</dt>
		<dd>boy</dd>
		<dd>bug</dd>
	</dl>
</div>

div {
	height: 100px;
	border: 1px solid red;
	overflow:auto;
}
* {
  box-sizing: border-box;
}

dl {
  margin: 0;
  padding: 24px 0 0 0;
}
dt {
  background: #B8C1C8;
  border-bottom: 1px solid #989EA4;
  border-top: 1px solid #717D85;
  color: #FFF;
  font: bold 18px/21px Helvetica, Arial, sans-serif;
  margin: 0;
  padding: 2px 0 0 12px;
  position: -webkit-sticky;
  position: sticky;
  top: -1px;
}
dd {
  font: bold 20px/45px Helvetica, Arial, sans-serif;
  margin: 0;
  padding: 0 0 0 12px;
  white-space: nowrap;
}