（参考サイト） https://www.granfairs.com/blog/staff/use-triangle-with-css

index.html

<h1 class="subtitle subtitle3">hoge</h1>

style.css

.subtitle3 {
  color: #a1c630;
  font-family: "mincho", serif;
  font-size: 18px;
  font-weight: bold;
  display: inline-block;
  min-width: 510px;
  position: relative;
  background: #f8f6f5;
  padding: 5px 7px;
}

.subtitle3:after {
  content: "";
  position: absolute;
  top: 0;
  right: 0;
  width: 0;
  height: 0;
  border-style: solid;
  border-color: transparent #fff transparent transparent;
  border-width: 40px 50px 0px 0px;
}