Ejemplo:
<div style="position: relative; padding-bottom: 56.25%; padding-top: 25px; height: 0;"> <iframe style="position: absolute; top: 0; left: 0; width: 100%; height: 100%;" width="560" height="315" src="https://www.youtube.com/embed/JC82Il2cjqA" frameborder="0" gesture="media" allow="encrypted-media" allowfullscreen></iframe> </div>