As a workaround you can use the fact percent vertical padding and margin are computed from the container width. It's quite a ugly solution and I don't know if you'll be able to use it but well, it works: http://jsfiddle.net/bFWT9/
<!DOCTYPE html>
<html>
<head>
<title></title>
</head>
<body>
<div>It works!</div>
</body>
</html>
html, body, div {
height: 100%;
}
body {
margin: 0;
}
div {
box-sizing: border-box;
margin-top: -75%;
padding-top: 75%;
background: #d35400;
color: #fff;
}