Javascript: Rewrite element content

How to change the content of an element...?

In the head tag:

<script type="text/javascript">
function giveWidth(e) {
 document.getElementById('wid').innerHTML = window.innerWidth;
}
</script>

In the body tag:

<p>Window width <a href="" onclick="giveWidth(event);return false">is</a> <span id="wid">500</span> pixels.<p>

The return false makes sure that the browser doesn't try to open a new window when you click on the link.