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

.cp_h1title {
  position: relative;
  border-bottom: 2px solid #bdbdbd ;
  padding: 3px 0;
}
.cp_h1title span {
  position: relative;
  color: #f8f8f8;
  background: #424242;
  padding: 5px 10px;
}
.cp_h1title span::after {
  position: absolute;
  content: '';
  top: 0;
  right: -10px;
  width: 0;
  border-width: calc(1em + 20px) 0 0 10px;
  border-style: solid;
  border-color: transparent transparent transparent #bdbdbd;
}