<div class="mk-iframe">
    <iframe src="https://www.mehmetkaplan.net/" allowfullscreen></iframe>
</div>

.mk-iframe {
  position: relative;
  overflow: hidden;
  width: 100%;
  padding-top: 56.25%;
}
.mk-iframe iframe {
  position: absolute;
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
  width: 100%;
  height: 100%;
}