When I design a page, I like to use some of the features of Microsoft IE that other browsers do not support. This wouldn't be a problem if a visitor's browser simply didn't display anything it didn't understand, but sometimes the offending code makes a mess of the page for the visitor. Is there an easy way for me to hide the code when the visitor doesn't have the specified browser or version?
The easiest way is to use the conditional comment tag introduced with Internet Explorer 5.0. The conditional comment is an extension of HTML's standard comment tag, which takes the form <!-- place comment here -->. The conditional comment tag works precisely the same way, except for the inclusion of an if-endif block placed within the tag. The purpose of the if-endif block is to provide the browser with the conditions under which to display the code within the tag. Specifically, it instructs the browser to render the HTML within the if-endif block only if the conditional statement is true.
Two terms you'll see in relation to conditional comments are uplevel browser and downlevel browser. Uplevel browsers include IE versions 5.0 and higher. Any other browser is considered downlevel. Uplevel browsers recognize conditional comments and process them logically. Downlevel browsers do not recognize them, and that lack of recognition results in their working as Microsoft intended them.
Here's an example of a conditional comment. In this case, a visitor using IE 5.0 or higher will see the text inside the paragraph tags. Visitors with other browsers will see neither the sentence nor any other part of the conditional statement.
<!-- [if IE 5]>
<p>Good to see you've upgraded to IE 5.</p>
<![endif]-->
Note that the sentence will appear in all sub-versions of IE 5: 5.0, 5.1, or 5.5. You may restrict the condition to the subversion by specifying it, which looks like this:
<!--[if IE 5.5]>
<p>Good to see you've upgraded to IE version 5.5.</p>
<![endif]-->
It's also possible to indicate a relative browser version�such as a browser version higher or lower than the one specified�by including an operator in the conditional statement. For example, the following condition would display the text if the visitor has not upgraded at least to IE 5.5:
<!--[if lt IE 5.5]>
<p>You need to upgrade to Internet
Explorer 5.5 or 6 to read this.
<![endif]-->
The possible operators are: lt for less than (in other words, prior to); gt for greater than; lte for less than or equal to; and gte for greater than or equal to. In addition, you can use the not operator�in the form of !�to specify code to display if the browser is not a specific version. The following example uses the not operator to inform visitors that they will only be able to view the text if they have IE 5.5:
<![if !IE 5.5]>
<p>Only users of Internet Explorer
5.5 will be able to see this text.</p>
<![endif]>
Note the difference between this code and the preceding examples. Here, the double hyphens do not follow the <! tag or the [endif]comment. A conditional comment without the double hyphens is called a downlevel-revealed comment; these comments appear in downlevel browsers, whereas comments that include the hyphens do not. Downlevel-revealed comments let you make a comment to visitors who do not use Internet Explorer at all, or who use IE versions earlier than 5.0.
Conditional comments work because IE 5.x has been programmed to process comment tags differently from other browsers. The uplevel browsers recognize the if-endif block, but all other browsers do not. If the comment tag starts and ends with the double hyphen, downlevel browsers will recognize it as a normal HTML comment and not display any of it, including the if-endif block or any contents inside that block; uplevel browsers, however, will render the if-endif block as instructed. If the comment does not contain the double hyphens, then downlevel browsers will ignore the comment tags as invalid but will render the HTML inside those tags. Uplevel browsers, however, will ignore them completely.
Although we've used simple paragraph containers in these examples, you are by no means restricted to these. You can include anything you wish inside the if-endif block, including scripts, graphics, specific IE features (such as marquees), and so on.