This very imperfect code will give you a fighting chance to determine the actual document mode msie is using. Also see Testing sites with Browser Mode vs. Doc Mode http://blogs.msdn.com/b/ie/archive/2010/10/19/testing-sites-with-browser-mode-vs-doc-mode.aspx

msie-document-mode.html

<script>var ccMsie = -1;</script>
<!--[if gte IE 6]><script>ccMsie = 6;</script><![endif]-->
<!--[if gte IE 7]><script>ccMsie = 7;</script><![endif]-->
<!--[if gte IE 8]><script>ccMsie = 8;</script><![endif]-->
<!--[if gte IE 9]><script>ccMsie = 9;</script><![endif]-->
<script>/*@cc_on @if (@_jscript_version == 10) ccMsie = 10; @end @*/</script>
<script>
(function(){
    function readVersion(s) {
		var ua = navigator.userAgent;
		var index = ua.indexOf(s);
		if (index == -1) {
			return -1;
		}
		return parseFloat(ua.substring(index + s.length + 1));
	}
	var msie = readVersion('MSIE');
	var compatMode = document.compatMode;
	var trident = readVersion('Trident');
	var documentMode = document.documentMode || -1;
	
	var browserModeString = 'Unknown';
	var documentModeString = 'Unknown';
	if (compatMode === 'CSS1Compat') {
		if (msie === documentMode && (trident === -1 || msie === trident + 4)) {
			browserModeString = 'IE' + documentMode;
		} else if (documentMode === trident + 4) {
			browserModeString = 'IE' + documentMode + ' Compat View';
		} else if (msie === trident + 4 || (msie === 7 && trident === -1)) {
			browserModeString = 'IE' + msie;
		}
		if (documentMode != -1 && (documentMode != msie || documentMode === 7)) {
			documentModeString = 'IE' + documentMode + ' Standards';
		} else {
			documentModeString = 'Standards';
		}
	} else {
		if (msie !== trident + 4 && !(msie === 7 && trident === -1)) {
			browserModeString = 'IE' + (trident + 4) + ' Compat View';
		} else {
			browserModeString = 'IE' + msie;
		}
		if (documentMode === 5) {
			documentModeString = 'IE5 Quirks';
		} else {
			documentModeString = 'Quirks';
		}
	}
	
	document.write(['<div style="position:absolute;top:50px;left:50px;background:#fff;color:#000;border:3px solid #000;padding:10px;font-size:16px;z-index:1000;">',
	'<pre>',
	'<b>Raw data</b>\n',
	'ccMsie       = ', ccMsie, '\n',
	'msie         = ', msie, '\n',
	'compatMode   = ', compatMode, '\n',
	'trident      = ', trident, '\n',
	'documentMode = ', documentMode, '\n',
	'\n',
	'<b>Interpretation</b>', '\n',
	'Browser Mode:  ', browserModeString, '\n',
	'Document Mode: ', documentModeString, '\n',
	'\n',
	'<b>Box model test</b>',
	'</pre>',
	'<div style="background:#000;width:100px;height:10px;"></div>',
	'<div style="background:#000;border-left:10px solid #333;border-right:10px solid #333;width:80px;height:10px;"></div>',
	'<div style="background:#333;border-left:10px solid #666;border-right:10px solid #666;padding:0 10px;width:60px;height:10px;"><div style="background:#000;height:10px;"></div></div>',
	'</div>'].join(''));
}());
</script>