Conditional comments are conditional statements interpreted by Microsoft Internet Explorer in HTML source code. Conditional comments can be used to provide and hide code to and from Internet Explorer.
Conditional comments in HTML first appeared in Microsoft's Internet Explorer 5 browser, although support has now been deprecated. In Internet Explorer 10 HTML conditional comments are not supported when the page is in standards mode (document mode 10). JScript conditional comments were introduced in Internet Explorer 4 and they continue to be supported in Internet Explorer 10, in standards mode or compatibility mode.
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]-->
There are two types of "conditional comments": downlevel revealed, and downlevel hidden.
The basic syntax of each type of comment is shown in the following table. The first comment shown is the basic HTML Comment, which is included for the purpose of comparison and to illustrate the different syntax used by each type of conditional comment.
|Comment type||Syntax or possible value|
|standard HTML comment||<!-- Comment content -->|
|downlevel-hidden||<!--[if expression]> HTML <![endif]-->|
|downlevel-revealed||<![if expression]> HTML <![endif]>|
The HTML shown inside the syntax block in each of the conditional comments denotes any block of HTML content, including script. Both types of conditional comment use a conditional expression to indicate whether the content inside the comment block should be parsed or ignored. The conditional expression is formed from a combination of feature, operator, and/or value, depending on the feature. The following table shows the supported features and describes the values each feature supports.
|IE||[if IE]||The string "IE" is a feature corresponding to the version of Internet Explorer used to view the Web page.|
|value||[if IE 7]||An integer or floating point numeral corresponding to the version of the browser. Returns a Boolean value of true if the version number matches the browser version. For more information, see Version vectors.|
|WindowsEdition||[if WindowsEdition]||Internet Explorer 8 on Windows 7. The string "WindowsEdition" is a feature corresponding to the edition of Microsoft Windows used to view the Web page.|
|value||[if WindowsEdition 1]||An integer corresponding to the edition of Windows used to view the Web page. Returns a Boolean value of true if the value matches the edition being used. For information about supported values and the editions they describe, see the pdwReturnedProductType parameter of the GetProductInfo function.|
|true||[if true]||Always evaluates to true.|
|false||[if false]||Always evaluates to false.|
The following table describes the operators that can be used to create conditional expressions.
|[if !IE]||The NOT operator. This is placed immediately in front of the feature, operator, or subexpression to reverse the Boolean meaning of the expression.|
|lt||[if lt IE 5.5]||The less-than operator. Returns true if the first argument is less than the second argument.|
|lte||[if lte IE 6]||The less-than or equal operator. Returns true if the first argument is less than or equal to the second argument.|
|gt||[if gt IE 5]||The greater-than operator. Returns true if the first argument is greater than the second argument.|
|gte||[if gte IE 7]||The greater-than or equal operator. Returns true if the first argument is greater than or equal to the second argument.|
|( )||[if !(IE 7)]||Subexpression operators. Used in conjunction with boolean operators to create more complex expressions.|
|&||[if (gt IE 5)&(lt IE 7)]||The AND operator. Returns true if all subexpressions evaluate to true|
||||[if (IE 6)|(IE 7)]||The OR operator. Returns true if any of the subexpressions evaluates to true.|
Below are two examples of a "downlevel hidden" conditional comment:
<!--[if IE 8]> <link href="ie8only.css" rel="stylesheet"> <![endif]-->
<!--[if lte IE 7]> <style type="text/css"> /* CSS here */ </style> <![endif]-->
The directive in the first example will let IE 8 read the specified CSS file, while IE 7 or older IE versions 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 newer (greater) or older (less) than a specified version.
Downlevel-revealed conditional comment 
Below is an example of a "downlevel revealed" conditional 'comment', which is not an (X)HTML comment at all, despite the misleading name, using the default Microsoft syntax:
<![if !IE]> <link href="non-ie.css" rel="stylesheet"> <![endif]>
This example shows content that should be exposed only to non-IE browsers, as the condition evaluates to "false" on IE (and hence the content is ignored), while the tags themselves are unrecognized (and hence ignored) on non-IE browsers. This is not valid HTML or XHTML.
Microsoft acknowledges this syntax is not standardized markup, intending these tags to be overlooked by other browsers and expose the content in the middle. In order to ensure compliance with W3C standards, some web developers use an alternative technique for downlevel-revealed conditional comments:
<!--[if !IE]>--> <link href="non-ie.css" rel="stylesheet"> <!--<![endif]-->
While somewhat confusing in structure, this specific syntax is valid (X)HTML and is useful for conditional sections intended specifically for non-IE browsers; 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 problem is easily solved by prepending "<!" to the initial "-->" as follows:
<!--[if gt IE 6]><!--> This code displays on non-IE browsers and on IE 7 or higher. <!--<![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.
Conditional comments in JScript 
<script> /*@cc_on document.write("You are using IE4 or higher"); @*/ </script>
There were also several predefined variables, though these cannot be relied on any longer as Microsoft altered the JScript engine of IE6 with XP SP3 and it now reports as:
@_jscript_version == 5.7
As a result, the way to detect IE 6 using conditional compilation can be seen below:
<script> /*@cc_on @if (@_jscript_version == 10) document.write("You are using IE10"); @elif (@_jscript_version == 9) document.write("You are using IE9"); @elif (@_jscript_version == 5.8) document.write("You are using IE8"); @elif (@_jscript_version == 5.7 && window.XMLHttpRequest) document.write("You are using IE7"); @elif (@_jscript_version == 5.6 || (@_jscript_version == 5.7 && !window.XMLHttpRequest)) 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>
See also 
- "About Conditional Comments". Microsoft Corporation.
- Microsoft - HTML5 Parsing in IE10
- "MSDN — About Conditional Comments". Archived from the original on 2007-01-03. Retrieved 2007-01-03.
- Valid downlevel-revealed conditional comments | 456 Berea Street
- "Conditional Compilation". Microsoft Corporation.
- Conditional Compilation Variables