<div class="envelope">
  <div class="header">
      Заголовок
  </div>
  <div class="content">
      Содержимое
  </div>
</div>

.envelope {
  background: rgb(185,185,185);
  width: 400px;
  height: 200px;
  display: flex;
  flex-direction: column;
  
  overflow: hidden;
  resize: both;
}

.header{
  position: relative;
  z-index: 1;
  margin-bottom: -25%;
  text-transform: uppercase;
  
  display: flex;
  align-items: center;
  justify-content: center;
}
.header::before{
  content: "";
  padding-bottom: 25%;
  display: block;
  width: 0px;
}

.content{
  overflow: hidden;
  background: lightgray;
  flex: 1;
  text-align: center;
}
.content::before {
  content: "";
  display:block;
  background: rgb(185,185,185);
  width: 100%;
  padding-top: 100%;
  transform: scaleY(.5) rotate(45deg);
  border-radius: 10%;
  border: 8px solid white;
  margin-left: -8px;
  margin-top: calc( -50% - 8px);
  margin-bottom: -15%;
}