<div class="cp_card">
コンテンツ
  <div class="cp_ribbon10">
    <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: 35px 5px 0;
  border: 2px solid #9575CD;
  box-shadow: 0 0 5px rgba(0, 0, 0, 0.3);
}
/*リボンの設定*/
.cp_ribbon10 {
  position: absolute;
  top: -6px;
  left: -6px;
  padding: 5px 40px 5px 10px;
  color: #ffffff;
  background: linear-gradient(-45deg,transparent,transparent 25%,#9575CD 0%,#9575CD);
}
/*リボンの影*/
.cp_ribbon10::before {
  position: absolute;
  content: '';
  top: 100%;
  left: 0;
  border: 3px solid transparent;
  border-top: 3px solid #5E35B1;
  border-right: 3px solid #5E35B1;
}