.ex {

  position: relative;

  margin-bottom: 50px;

}

​

.bg {

  position: absolute;

  background: red;

  width: 200px;

  height: 200px;

}

​

.bfh {

  -webkit-backface-visibility:hidden;

}

​

​

.mask {

  position: relative;

  width: 200px;

  height: 200px;

  border-radius: 50px;

  background: transparent;

  overflow: hidden;

}