To create Youtube embedded code with LOOP, first create a playlist and add the video you wanted to have as loop. Use below code, change your video link (url) and playlist link (url).

<iframe width="100%" height="425" src="https://www.youtube.com/embed/U-Vs550CRc4?autoplay=1&showinfo=0&loop=1&list=PLyU5X-W7kMhMj9oBTnP58QS7bV9AVkZu4&rel=0" frameborder="0" allowfullscreen></iframe>

To make your Youtube video as responsive use below HMTL & CSS.

CSS

.videoWrapper {
position: relative;
padding-bottom: 56.25%; /* 16:9 */
padding-top: 25px;
height: 0;
}
.videoWrapper iframe {
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
}

HTML

<div class="videoWrapper">
<iframe width="100%" height="425" src="https://www.youtube.com/embed/U-Vs550CRc4?autoplay=1&showinfo=0&loop=1&list=PLyU5X-W7kMhMj9oBTnP58QS7bV9AVkZu4&rel=0" frameborder="0" allowfullscreen></iframe>
</div>