Internet Explorer 5 through IE 9 supports conditional comments.
IE is the only browser that understands or uses conditional comments (it’s also the only one that needs to!).
What are conditional comments? How do you use them?
<!--[if IE]> <p>You are using Internet Explorer. Please stop.</p> <![endif]-->
The conditional comments starts & ends with standard HTML comment syntax:
-->. Every web browser except IE will see that & ignore everything inside it. IE, however, will see a comment & then keep looking. If it see the stuff in square brackets—
[if IE]> &
<![endif] in this example—it will use or display the code inside the comment1. It can also hide stuff instead of show, & do more than that. To learn more, start with these articles.
Start with Wikipedia (as with so many things!): Conditional comment. Explanation, examples, references.
This article, from Microsoft, does a good job explaining what they are & how to use them: About Conditional Comments.
For more info, see Conditional comments, from the great QuirksMode website.
If IE does not see code that indicates that it's dealing with a conditional comment—if the stuff, in other words, inside the square brackets is missing—then it treats the code as commented out & ignores it, just like every other web browser in the world. ↩