I'd suggest getting into one of the JS libraries out there. They ensure compatibility so you can get up and running really fast. jQuery and DOJO are both really great. To do what you're trying to do in jQuery, for example, it would go something like this:
<script type="text/javascript" language="JavaScript">
$.ajax({
url: "x.html",
context: document.body,
success: function(response) {
$("#yourDiv").html(response);
}
});
</script>