How to make a YouTube embed video responsive using CSS

Quick snippet how to make a youtube embed video responsive by CSS only.

The easiest way, using viewport units (IE9+)

iframe {
    width: 100vw; 
    height: 56.25vw; /* 100/56.25 = 560/315 = 1.778 */
    background:red;  
}

CSS2 style fix using a CSS trick and wrapper with class .aspect-ratio:

    .aspect-ratio {
      position: relative;
      width: 100%;
      height: 0;
      padding-bottom: 56%; /* The height of the item will now be 56% of the width. */
    }
 
    /* Adjust the iframe so it's rendered in the outer-width and outer-height of it's parent */
    .aspect-ratio iframe {
      position: absolute;
      width: 100%;
      height: 100%;
      left: 0;
      top: 0;
    }