CSS aspect ratio

HTML

<div id="container">
    <div id="element">
        some text
    </div>
</div>

CSS

#container {
    position: relative;
   /* width: 100%;*/
}
#container:before {
    content: "";
    display: block;
    padding-top: 100%;
}
#element {
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    background-color: silver;
}