Conditional comment
From Wikipedia, the free encyclopedia
Conditional comments are proprietary conditional statements interpreted by Microsoft Internet Explorer in HTML source code. Conditional comments first appeared in Microsoft's Internet Explorer 5 browser and is supported through at least version 8.[1]
Conditional comments can be used to provide and hide code to and from Internet Explorer.
There are two types of "conditional comment", downlevel revealed, and downlevel hidden.
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[edit] Examples
Here is a simple example that demonstrates how conditional comments work.
<!--[if IE 6]> <p>You are using Internet Explorer 6.</p> <![endif]--> <![if !IE]> <p>You are not using Internet Explorer.</p> <![endif]>
[edit] Downlevel-hidden conditional comment
Below are two examples of a "downlevel hidden" conditional comment:
<!--[if IE 7]> <link href="ie7only.css" rel="stylesheet"> <![endif]-->
or
<!--[if lte IE 7]> <style type="text/css"> /* [[CSS]] here */ </style> <![endif]-->
The tag in the first example will let IE 7 read the specified CSS file while IE 6 or less will ignore it. Browsers other than IE will also ignore it because it looks like a standard HTML comment. The tag in the second example will let IE versions 5.0 through 7 read the internal CSS style. With different uses of this tag you can also single out IE 6, IE 5, or versions of IE that are greater or lesser than a specified version.
[edit] Downlevel-revealed conditional comment
Below is an example of a "downlevel revealed" conditional comment:
<![if !IE]> <link href="non-ie.css" rel="stylesheet"> <![endif]>
This is recommended by Microsoft for when the content should be exposed to non-IE browsers.
Microsoft acknowledges this method is not standardized markup for comments,[2] intending these tags to be overlooked by other browsers and expose the content in the middle. Some web developers use an alternative technique[3] for downlevel-revealed conditional comments in order to only use markup that appears less irregular.
<!--[if !IE]>--> <link href="non-ie.css" rel="stylesheet"> <!--<![endif]-->
This method has the drawback that if the condition evaluates to true (for example, if writing code meant to display on non-IE browsers and on some versions of IE), IE will then display the "-->" present before the HTML content. This is easily solved by prepending "<!" to the initial "-->" as follows:
<!--[if gt IE 6]><!--> <p>This code displays on non-IE browsers and on IE 7.</p> <!--<![endif]-->
The extra "<!" is ignored by non-IE browsers; it is also ignored by IE regardless of the condition because if false, everything within the conditional comment is ignored, and if true, the resulting tag "<!-->" is unrecognized and therefore ignored.
While this method is functional in current versions of Internet Explorer, there is no guarantee that future versions will continue to operate this way.
[edit] Conditional Comment in JScript
Starting with Internet Explorer 4, there exists a similar proprietary mechanism for adding conditional comments within JScript, known as Conditional Compilation.[4]
Code examples:
<script type="text/javascript">
/*@cc_on
document.write("You are using IE4 or higher");
@*/
</script>
There are also several predefined variables[1].
<script type="text/javascript">
/*@cc_on
@if (@_jscript_version > 5.7)
document.write("You are using IE8+");
@elif (@_jscript_version == 5.7)
document.write("You are using IE7");
@elif (@_jscript_version == 5.6)
document.write("You are using IE6");
@elif (@_jscript_version == 5.5)
document.write("You are using IE5.5");
@else
document.write("You are using IE5 or older");
@end
@*/
</script>
[edit] See also
[edit] References
- ^ "About Conditional Comments". Microsoft Corporation. http://msdn2.microsoft.com/en-us/library/ms537512.aspx.
- ^ "MSDN — About Conditional Comments". Archived from the original on 2007-01-03. http://web.archive.org/web/20070103163310/http://msdn.microsoft.com/workshop/author/dhtml/overview/ccomment_ovw.asp. Retrieved on 2007-01-03.
- ^ Valid downlevel-revealed conditional comments | 456 Berea Street
- ^ "Conditional Compilation". Microsoft Corporation. http://msdn2.microsoft.com/en-us/library/ahx1z4fs(VS.80).aspx.
