Thanks for your help guys, again. This problem kind of flopped cross two threads lol.
I ended up finding this which worked for what I wanted, though it ended up being the wrong thing for me to be doing anyway.
Someone else may find it useful though.
I made an iframe on a new HTML document and used this script to put the HTML5 page inside of it, AND fill the page leaving a dedicated "margin" at the bottom:
<iframe id="frame" src="http://google.com/" width="100%" frameborder="0" marginheight="0" marginwidth="0"></iframe>
<script type="text/javascript">
function resizeIframe() {
var height = document.documentElement.clientHeight;
height -= document.getElementById('frame').offsetTop;
// not sure how to get this dynamically
height -= 20; /* whatever you set your body bottom margin/padding to be */
document.getElementById('frame').style.height = height +"px";
};
document.getElementById('frame').onload = resizeIframe;
window.onresize = resizeIframe;
</script>
~Sol