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* { margin: 0; padding: 0; }
.video { width: 100%; }

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>

jQuery(function($){
  
  resizeIfame(".video iframe");
  
});
/* ========================================================================= */
/* RE FUNCTION */
/* ========================================================================= */
function resizeIfame(frame) {

    var oldWidth = $(frame).width();
    var oldHeight = $(frame).height();
    var propotion = oldHeight / oldWidth;
    var newHeight;

    $(frame).width('100%');
    newHeight = $(frame).width() * propotion;
    $(frame).height(newHeight);

    $(window).resize(function() {
        $(frame).width('100%');
        newHeight = $(frame).width() * propotion;
        $(frame).height(newHeight);
    });
}

<div class="video">
  <iframe width="560" height="315" src="https://www.youtube.com/embed/TWluKrCIBSY?autoplay=1" frameborder="0" allowfullscreen></iframe>
</div>