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<div class="ribbon">
  <span>-70%</span>
</div>

.ribbon {
  margin: 100px auto;
  width: 110px;
  height: 110px;
  display: block;
  position: relative;
  overflow: hidden;
}
.ribbon span {
  width: 150px;
  height: 34px;
  top: 20px;
  right: -40px;
  position: absolute;
  display: block;
  background: #FF0000;
  color: #333;
  font-family: arial;
  font-size: 18px;
  color:white;
  text-align: center;
  line-height: 34px;
  transform: rotate(45deg);
  -webkit-transform: rotate(45deg);
  -ms-transform: rotate(45deg);
}