<div class="cp_card">
コンテンツ
  <div class="cp_ribbon08">
    <div class="cp_ribbon">Ribbon</div>
  </div>
</div>

.cp_card {
  position: relative;/*リボンをかけたいコンテンツボックスにセット*/
  width: 300px;
  height: 300px;
  background: #ffffff;
  margin: auto;
  padding: 5px;
  box-shadow: 0 0 5px rgba(0, 0, 0, 0.3);
}
/*リボンの位置*/
.cp_ribbon08 {
  position: absolute;
  top: 20px;
  left: -8px;
}
/*リボン内のテキスト*/
.cp_ribbon08 .cp_ribbon {
  position: relative;
  display: flex;
  align-items: center;
  width: 100%;
  height: 40px;
  padding: 0 0 0 10px;
  color: #ffffff;
  background: #4CAF50;
}
/*擬似要素の共通設定*/
.cp_ribbon08:before,
.cp_ribbon08:after {
  position: absolute;
  content: '';
}
.cp_ribbon08:before {/*リボンの影部分*/
  top: -8.5px;
  left: 0.1px;
  border-bottom: 9px solid #2E7D32;
  border-left: 9px solid transparent;
}
/**/
.cp_ribbon08:after {/*リボン下のカット部分*/
  top: 0;
  right: -20px;
  width: 0;
  height: 0;
  border-top: 20px solid transparent;
  border-bottom: 20px solid transparent;
  border-left: 10px solid #4CAF50;
}