Autosizing images based on aspect ratio

HTML

<div class="container">
    <h4>Portrait Style</h4>
    <img src="http://placekitten.com/240/300">
</div>

CSS

.container img {
    display: block;
    width: 100%;
    height: 60vh;
    /*object-fit: cover; // doesn't work in Internet Explorer */
}