/* HTML: <div class="ribbon">Your text content</div> */
.ribbon {
  font-size: 28px;
  font-weight: bold;
  color: #fff;
}
.ribbon {
  --f: .5em; /* control the folded part */
  
  position: absolute;
  top: 0;
  right: 0;
  line-height: 1.8;
  padding-inline: 1lh;
  padding-bottom: var(--f);
  border-image: conic-gradient(#0008 0 0) 51%/var(--f);
  clip-path: polygon(
    100% calc(100% - var(--f)),100% 100%,calc(100% - var(--f)) calc(100% - var(--f)),var(--f) calc(100% - var(--f)), 0 100%,0 calc(100% - var(--f)),999px calc(100% - var(--f) - 999px),calc(100% - 999px) calc(100% - var(--f) - 999px));
  transform: translate(calc((1 - cos(45deg))*100%), -100%) rotate(45deg);
  transform-origin: 0% 100%;
  background-color: #BD1550; /* the main color  */
}