HTML条件注释和javascript条件注释

<!--[if IE]>
This content is actually inside an HTML comment.
It will only be displayed in IE.
<![endif--]>
<!--[if IE6]>
This content is actually only be displayed by IE 6 and later.
<![endif]-->
<!--[if !IE]><-->
This is normal HTML content,but IE will not display it because of
the comment above and the comment below.
<!--><![endif]-->

This is normal content,displayed by all browsers.
<script>
/*@cc_on

 @if(@_jscript)
  //This code is inside a JS comment but is executed in IE.
  //This code is inside a conditional comment,which is also a
  //regular Javascript comment.IE runs it but other browsers ignore it.
  alert("You are using Internet Explorer");
 @else*/
  //This code is no longer inside a Javascript comment,but is still
  //inside the IE conditional comment. This means that all browsers
  //excepte IE will run this code.
  alert("you are not using Internet Explorer");
 /*@end
 @*/
 </script>

有些用处，但非标准。