.yt_box{
  position: relative;
  max-width: 800px; /* wie breit soll das Video maximal sein */
  margin: auto;
}
.yt_box:before{
  content: "";
  display: block;
  padding-top: 56%; /* 16:9 Format */
}
.yt_box iframe {
  width: 100%;
  height: 100%;
  position: absolute;
  top: 0;;
  left: 0;
  right: 0;
  bottom: 0;
}