<div>abcd</div>
<script>
var div = document.getElementsByTagName('div')[0];
alert(div.currentStyle.width);
alert(div.currentStyle.height);
</script>

<div style="width:100px;height:50px;">abcd</div>
<script>
var div = document.getElementsByTagName('div')[0];
alert(div.currentStyle.width);
alert(div.currentStyle.height);
</script>

<style>
div {
width: 100px;
height: 50px;
}
</style>
<div>abcd</div>
<script>
var div = document.getElementsByTagName('div')[0];
alert(div.currentStyle.width);
alert(div.currentStyle.height);
</script>