This may be old news, but I just found out now.
Internet Explorer 10 does not support conditional comments in Standards mode!
<!--[if IE 6]> <link href="ie6only.css" rel="stylesheet" /> <![endif]-->
Since it’s a valid HTML comment, standards-compliant browsers (i.e. anything other than IE) just ignored it.
While that’s good for Web developers and the progress of the Web as a platform, it means I can’t “torment” IE users anymore using them. Oh well, at least JScript conditional comments are still supported in Standards mode.