Html – Detect aspect ratio – HTML5 video

Just create a wrapper <div> with a percentage value for padding-bottom, like this:

.demoWrapper {
  padding: 10px;
  background: white;
  box-sizing: border-box;
  resize: horizontal;
  border: 1px dashed;
  overflow: auto;
  max-width: 100%;
  height: calc(100vh - 16px);
}

div {
  width: 100%;
  padding-bottom: 75%;
  background: gold; /** <-- For the demo **/
}

<div class="demoWrapper">
  <div></div>
</div>

It will result in a <div> with height equal to 75% of the width of its container (a 4:3 aspect ratio).

This relies on the fact that for padding :

The percentage is calculated with respect to the width of the generated box's containing block [...] (source: w3.org, emphasis mine)

Padding-bottom values for other aspect ratios and 100% width :

aspect ratio  | padding-bottom value
--------------|----------------------
    16:9      |       56.25%
    4:3       |       75%
    3:2       |       66.66%
    8:5       |       62.5%

Placing content in the div :

In order to keep the aspect ratio of the div and prevent its content from stretching it, you need to add an absolutely positioned child and stretch it to the edges of the wrapper with:

div.stretchy-wrapper {
  position: relative;
}

div.stretchy-wrapper > div {
  position: absolute;
  top: 0; bottom: 0; left: 0; right: 0;
}

Here's a demo and another more in depth demo

Update 2016:

Modern browser behave much better. All you should need to do is to set the image width to 100% (demo)

.container img {
   width: 100%;
}

Since you don't know the aspect ratio, you'll have to use some scripting. Here is how I would do it with jQuery (demo):

CSS

.container {
    width: 40%;
    height: 40%;
    background: #444;
    margin: 0 auto;
}
.container img.wide {
    max-width: 100%;
    max-height: 100%;
    height: auto;
}
.container img.tall {
    max-height: 100%;
    max-width: 100%;
    width: auto;
}​

HTML

<div class="container">
 <img src="http://i48.tinypic.com/wrltuc.jpg" />
</div>
<br />
<br />
<div class="container">
 <img src="http://i47.tinypic.com/i1bek8.jpg" />
</div>

Script

$(window).load(function(){
 $('.container').find('img').each(function(){
  var imgClass = (this.width/this.height > 1) ? 'wide' : 'tall';
  $(this).addClass(imgClass);
 })
})