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.responsiveContainer {
  position: relative;
  padding-bottom: 56%;
  height: 0;
  overflow: hidden;
}

.responsiveContainer iframe {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
}

<div class="responsiveContainer">
  <!-- iframe-Code von Google Maps -->
</div>