How to implement constant width to height ratio with CSS and HTML

Brief explanation and code sample:

Given an element of variable width, it will ensure its height remains proportionate in a responsive fashion (i.e., its width to height ratio remains constant).

padding-top on the ::before pseudo-element causes the height of the element to equal a percentage of its width. 100% therefore means the element's height will always be 100% of the width, creating a responsive square.

This method also allows content to be placed inside the element normally.

HTML

<div class="constant-width-to-height-ratio"></div>

CSS

.constant-width-to-height-ratio {
  background: #333;
  width: 50%;
}
.constant-width-to-height-ratio::before {
  content: '';
  padding-top: 100%;
  float: left;
}
.constant-width-to-height-ratio::after {
  content: '';
  display: block;
  clear: both;
}

Happy Coding!