'if compare
http://msdn.microsoft.com/en-us/library/ms537512.aspx
!	[if !IE]	The NOT operator. This is placed immediately in front of the feature, operator, or subexpression to reverse the Boolean meaning of the expression.
lt	[if lt IE 5.5]	The less-than operator. Returns true if the first argument is less than the second argument.
lte	[if lte IE 6]	The less-than or equal operator. Returns true if the first argument is less than or equal to the second argument.
gt	[if gt IE 5]	The greater-than operator. Returns true if the first argument is greater than the second argument.
gte	[if gte IE 7]	The greater-than or equal operator. Returns true if the first argument is greater than or equal to the second argument.
( )	[if !(IE 7)]	Subexpression operators. Used in conjunction with Boolean operators to create more complex expressions.
&	[if (gt IE 5)&(lt IE 7)]	The AND operator. Returns true if all subexpressions evaluate to true
|	[if (IE 6)|(IE 7)]	The OR operator. Returns true if any of the subexpressions evaluates to true.


'browser
http://rafael.adm.br/css_browser_selector/

<?php
	$msie = strpos($_SERVER["HTTP_USER_AGENT"], 'MSIE') ? true : false;
	$firefox = strpos($_SERVER["HTTP_USER_AGENT"], 'Firefox') ? true : false;
	$safari = strpos($_SERVER["HTTP_USER_AGENT"], 'Safari') ? true : false;
	$chrome = strpos($_SERVER["HTTP_USER_AGENT"], 'Chrome') ? true : false;
?>

<!--[if IE 8]>
<p>Welcome to Internet Explorer 8.</p>
<![endif]-->


    ie - Internet Explorer (All versions)
    ie8 - Internet Explorer 8.x
    ie7 - Internet Explorer 7.x
    ie6 - Internet Explorer 6.x
    ie5 - Internet Explorer 5.x
    gecko - Mozilla, Firefox (all versions), Camino
    ff2 - Firefox 2
    ff3 - Firefox 3
    ff3_5 - Firefox 3.5
    ff3_6 - Firefox 3.6 new
    opera - Opera (All versions)
    opera8 - Opera 8.x
    opera9 - Opera 9.x
    opera10 - Opera 10.x
    konqueror - Konqueror
    webkit or safari - Safari, NetNewsWire, OmniWeb, Shiira, Google Chrome
    safari3 - Safari 3.x
    chrome - Google Chrome
    iron - SRWare Iron