CSS を使用して角が折り返されたレスポンシブ リボンを作成する方法

<code class="css">.container {
  width: 200px;
  height: 200px;
  position: relative;
  margin: 20px;
  overflow: hidden;
}

.box {
  width: 100%;
  height: 100%;
  position: absolute;
  top: 0;
  left: 0;
  opacity: 0.8; /* for demo purpose */
}

.stack-top {
  height: 30px;
  z-index: 9;
  margin: 40px; /* for demo purpose */
  transform: rotateY(0deg) rotate(45deg); /* needs Y at 0 deg to behave properly*/
  transition: transform 2s;
  color: #fff;
}</code>

<code class="html"><div class="container">
  <div class="box" style="background: #fffff3;"></div>
  <div class="box stack-top" style="background: #242424;">      1Month</div>
</div></code>