Conditional comments can be used to customize code for different versions of Microsoft Internet Explorer. For example, different HTML classes, script tags, or stylesheets can be provided. Conditional comments are supported in Internet Explorer versions 5 through 9. Older and newer Internet Explorer versions, and all non-IE browsers, are considered "downlevel" and treat conditional comments as ordinary HTML comments.
Downlevel-hidden comments work by encapsulating the entire content within what appears to be a normal HTML comment. Only IE 5 through 9 will still read it as a conditional comment, and they will hide or display the content accordingly. In other browsers the content will be hidden.
<!--[if IE]> Revealed in IE 5 through 9. Commented out and hidden in all other browsers. <![endif]--> <!--[if lt IE 8]> Revealed only in specified versions of IE 5-9 (here, IE less than 8). <![endif]--> <!--[if !IE]> Revealed in no browsers. Equivalent to a regular HTML comment. <![endif]--> <!-- For purposes of comparison, this is a regular HTML comment. -->
These are slightly different than downlevel-hidden comments: only the conditional comment itself is contained within the normal comment syntax. Browsers which do not support conditional comments will simply ignore them and display the rest of the content between them.
<!--[if IE]>--> The HTML inside this comment is revealed in IE 5-9, and in all other browsers. <!--<![endif]--> <!--[if IE 9]>--> This is revealed in specified versions of IE 5-9, and in all other browsers. <!--<![endif]--> <!--[if !IE]>--> This is not revealed in IE 5-9. It's still revealed in other browsers. <!--<![endif]-->