.iframe-container{
  position: relative;
  width: 100%;
  padding-bottom: 56.25%; 
  height: 0;
}
.iframe-container iframe{
  position: absolute;
  top:0;
  left: 0;
  width: 100%;
  height: 100%;
}
div.a {
  font-size: 18px;
}

div.b {
  font-size: large;
}

div.c {
  font-size: 150%;
}