<!DOCTYPE html>
<html>
<head>
<title>div_height_equal_width</title>
<style type="text/css">
#div1{
width:50%
background-color:lavender
margin:0 auto
}
</style>
<script type="text/javascript">
window.onload = function(){
var div1 = document.getElementById("div1")
var width = div1.offsetWidth
div1.style.height = width+"px"
}
</script>
</head>
<body>
<div id="div1"></div>
</body>
</html>