Resize iFrame Responsive and Proportionally

If you are using HTML iFrame on your code you can get pretty annoyed dealing with your iframe height. Here you can get a really simple solution using CSS inline or external.
Basically you need to put your iFrame inside a Div, and with CSS give them both properties.
Here's the code

The CSS

.iframe-container {
 overflow: hidden;
 padding-top: 56.25%;/*(in this case I used 16:9 proportion 9÷16×100=56.25)*/
 position: relative;
 }
 .iframe-container iframe {
 border: 0;
 height: 100%;
 left: 0;
 position: absolute;
 top: 0;
 width: 100%;
 }

The HTML

 <div class="iframe-container">
 <iframe src="https://yourMovieSourceUrl.com" allowfullscreen></iframe>
 </div>