Many people were introduced to conditional comments with the introduction of IE7 and their grass-roots call to action to stop using "CSS Hacks" to target specific IE browser versions.
Conditional comments are totally valid markup (for HTML as well as XHTML doctypes) that cause IE browsers running in Windows (the old Mac versions of IE ignored them completely) to parse the contents of the commented markup.
The following code shows how a conditional comment can be used to match "all versions of IE less than version 7" and include an extra css file and an extra div in the content of the page:
Because of the start HTML comment code (<!--), everything until the close HTML comment code (-->) is treated as a comment for non matching IE Windows browsers. The code that is actually rendered by the browser is different when viewed in IE Windows browsers older than IE7 and any other browser:
CODE --> rendered pre-IE7 browsers
<html> <head> <link href="default.css" type="text/css" rel="stylesheet" /> <link href="iexplore.css" type="text/css" rel="stylesheet" /> </head> <body> <h1>Testing</h1> <div>I'm an old IE running in Windows</div> </body> </html>