Serving IE users other content

Say you want to serve Internet Explorer users something different than users using another browser, such as a "Get Firefox" banner, or just do not want IE users to view your site because you know IE will screw it up anyway. You could then use user agent detection, either clientside (e.g. JavaScript) or serverside (e.g. PHP). But then some Opera user or someone using the Firefox UA Switcher Extension comes to your site - and will get the IE content, will think "Huh! I don't use IE. Weird site." and he'll close your site.

Fortunately, there is an alternative to UA detection, called conditional comments. It are comments, but only when the condition does or doesn't match. For example, IE might parse the content in the comment if the condition matches. They are only supported by IE, because they are a non-standard thing (why would you need them for other browsers anyway). I personally use them to serve IE users an extra CSS sheet containing bugfixes, and I also once made a "Get Firefox" banner for IE users.

Let's look at some code:

<![if !IE]>
	You at least use a non-IE browser, and maybe even Firefox!
	If you place things here, IE users won't see them.
<![endif]>
<!--[if IE]>
	You should use <a href="http://www.getfirefox.com/">Firefox</a>!
	IE users will see this instead.
<![endif]-->

You at least use a non-IE browser, and maybe even Firefox! If you place things here, IE users won't see them.

As you can see, the first conditional comment will work for browsers that are not IE, which works because it isn't actually a comment, but IE will see it as being one and ignore what is between the two tags. The second block is actually a comment, although it doesn not look like it. If IE sees the condition matches, it will parse the content in the comment. You can also use version checks in the conditions.