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

.cp_h1title {
  position: relative;
  text-align: center;
  border: 1px solid #bf92b8;
  margin-left: 20px;
}
.cp_h1title::before {
  position: absolute;
  content: attr(title);
  top: -15px;
  left: -20px;
  background: #ffffff;
  font-size: 50%;
  padding: 0 10px;
}