HTML:
<div class="videocontainer">
<iframe src="https://www.youtube-nocookie.com/embed/XXXXXXXX?rel=0" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen class="fullvideo"></iframe></div>
CSS:
.videocontainer {
position: relative;
width: 100%;
height: 0;
padding-bottom: 56.25%;
}
.fullvideo {
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
}