<div class="cp_card">
コンテンツ
  <div class="cp_ribbon04">
    <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);
}
/*リボンの部分*/
.cp_ribbon04 {
  width: 104px;
  height: 103px;
  overflow: hidden;
  position: absolute;
  bottom: -4px;
  left: -4px;
}
.cp_ribbon04 .cp_ribbon {
  font-size: 1em;
  line-height: 1.2em;
  position: relative;
  bottom: -50px;
  left: -33px;
  width: 150px;
  padding: 7px 0;
  transform: rotate(45deg);
  text-align: center;
  letter-spacing: 0.5px;
  color: #ffffff;
  background-color: #9C27B0;
  box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
}
.cp_ribbon04 .cp_ribbon::before,
.cp_ribbon04 .cp_ribbon::after {
  position: absolute;
  top: -4px;
  content: '';
  border-width: 4px 4px 0 4px;
  border-style: solid;
  border-color: transparent transparent transparent #6A1B9A;
}
.cp_ribbon04 .cp_ribbon:before {
  left: 0;
}
.cp_ribbon04 .cp_ribbon:after {
  right: 4px;
  top: -4px;
  border-width: 0 4px 4px 4px;
  border-color: transparent transparent #6A1B9A transparent;
}