<h1 class="cp_h1title">見出しテキスト</h1>

.cp_h1title {
  position: relative;
  border: solid 2px #2196F3;
  border-radius: 3px 0 3px 0;
  width: calc(100% - 26px);
  margin-left: 13px;
}
.cp_h1title:before,.cp_h1title:after {
  content: '';
  position: absolute;
  width:10px;
  height: 10px;
  border: solid 2px #2196F3;
  border-radius: 20% 50% 20% 50%;
}
.cp_h1title:after {
  top: -13px;
  left: -13px;
}
.cp_h1title:before {
  bottom: -13px;
  right: -13px;
}