Situations with Iframes:
- Adding a remote video via the Insert Media button adds a Drupal-Media tag, that is actually in an iFrame that has no ability to change width and height, or add a Bootstrap Class that makes the video responsive. The Stiff Little Fingers video illustrates this.
2. If you add an iframe via the iframe button at the top of Ckeditor, you can then go into the Source and add attribute width="100%" height will remain a fixed height, and you'll have to play around with the height to get the size video you want.
<div class="embed-responsive">
<iframe class="embed-responsive-item" src="https://www.youtube.com/embed/zpOULjyy-n8?rel=0" allowfullscreen=""> </iframe>
</div>