<!DOCTYPE HTML>
<html>
<head>
<style>
#map {
width: 200px;
}
</style>
</head>
<body>
<div id="map">Map</div>
<script>
window.onload = function () {
var map = getComputedStyle(document.getElementById('map'), '');
alert(map.width);
}
</script>
</body>
</html>