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HTML

              
                .wrapper-box.u-centercontent
	.aspectratio(data-ratio="16:9")
		div.u-centercontent
			p.u-centertext This text should appear in a green box which has a 16:9 aspect-ratio.<br />The green box itself is contained in a blue box of at least 500px in height.

              
            
!

CSS

              
                /*

There's a bug in Firefox where percentage based padding is not calculated correctly for an element wrapped inside a flexboxed element.

Normally percentage based padding is calculated with respect to the width of said element.

In Firefox however, the height of .aspectratio is always 281.25px, which happens to be 56.25% of the *height* of its parent element.

Try toggling the "display: flex" line (used to center the content vertically), and resize your browser window. You'll see.

Related Blog Post: https://www.bram.us/2017/07/30/vertical-marginspaddings-and-flexbox-a-quirky-combination/

Firefox Bug Report: https://bugzilla.mozilla.org/show_bug.cgi?id=1315669

*/

body
	background white
 	width 100%
	height 100%
	
// Utility class to center content using flexbox
.u-centercontent
	display flex
	align-items center
	justify-content center
	
// Utiltity class to center text
.u-centertext
	text-align center

// The wrapper box
.wrapper-box
	width 100%
	height 500px
	
	// For demo purposes, color me blue
	background blue
	
	> .aspectratio

		// For demo purposes, color my contained ar box green
		background lime
	
// CSS to create boxes with fixed aspect ratios.
// See https://www.bram.us/2017/06/16/aspect-ratios-in-css-are-a-hack/ for more info
.aspectratio
	width 100%
	height 0
	position relative
	
	&[data-ratio="16:9"]
		padding-top: 56.25%;
	
	&[data-ratio="4:3"]
		padding-top: 75%;
		
	> *
		position absolute
		top 0
		left 0
		width 100%
		height 100%
              
            
!

JS

              
                
              
            
!
999px

Console