<div class="embed-container">
    <iframe src="https://www.youtube.com/embed/9d8wWcJLnFI" frameborder="0" allowfullscreen></iframe>
</div>

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