Add the iframe video in a div named video-content:
<div class="video-content">
<iframe...></iframe>
</div>
Add the css:
.video-content {
position:relative;
padding-bottom:56.25%;
padding-top:30px;
height:0;
overflow:hidden;
}
.video-content iframe, .video-container object, .video-container embed {
position:absolute;
top:0;
left:0;
width:100%;
height:100%;
}