How To Make a Responsive 100% Width YouTube iFrame Embed

The key to creating a responsive YouTube embed is with padding and a container element, which allows you to give it a fixed aspect ratio. You can also use this technique with most other iframe-based embeds, such as slideshows.

Here is what a typical YouTube embed code looks like, with fixed width and height:

<iframe src=”//www.youtube.com/embed/yCOY82UdFrw” width=”560″ height=”315″ frameborder=”0″ allowfullscreen=”allowfullscreen”></iframe>

It would be nice if we could just give it a 100% width, but it won’t work as the height remains fixed. What you need to do is wrap it in a container like so (note the class names and removal of the width and height):
<div class=”container”><iframe class=”video” src=”//www.youtube.com/embed/yCOY82UdFrw” frameborder=”0″ allowfullscreen=”allowfullscreen”></iframe></div>
And use the following CSS:

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

How this works: The container element is given a zero height and a percentage bottom padding. The percentage bottom padding is a percentage of the container width, so that gives it a fixed aspect ratio. But in order to get the iframe to show up inside the zero-height container, you need to make the container relative and the iframe absolute, positioned inside the div.

https://www.h3xed.com/web-development/how-to-make-a-responsive-100-width-youtube-iframe-embed