<h1 class="cp_h1title"><span title="サブテキスト">見出しテキスト</span></h1>

.cp_h1title {
  position: relative;
  display: flex;
  justify-content: center;/*左右中央配置*/
  padding: 10px;
}
.cp_h1title::before,
.cp_h1title::after {
  position: absolute;
  left: 0;
  content: '';
  width: 100%;
  height: 6px;/*ボーダー全体の高さ*/
  border-color: #333;
  border-style: solid;
}
.cp_h1title::before {/*上ボーダー*/
  top: 0;
  border-width:2px 0 1px 0;
}
.cp_h1title::after {/*下ボーダー*/
  bottom: 0;
  border-width:1px 0 2px 0;
}
.cp_h1title span {
  position: relative;
  display: flex;
  justify-content: center;/*左右中央配置*/
}
.cp_h1title span::before {/*サブテキスト*/
  position: absolute;
  content: attr(title);
  top: -1em;
  background: #ffffff;
  font-size: 50%;
  white-space: nowrap;
  padding: 0 10px;
}