Conditional comment
Conditional comments are conditional statements interpreted by Microsoft Internet Explorer versions 5 through 9 in HTML source code. They can be used to provide and hide code to and from these versions of Internet Explorer. Conditional comments are not supported in Internet Explorer 10 and 11.
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. JScript conditional comments were introduced in Internet Explorer 4, and they continued to be supported in Internet Explorer 10, in standards mode or compatibility mode.
Examples
Here is a simple example that demonstrates how conditional comments work.Syntax
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 | <!--> HTML <!--> |
| downlevel-revealed | <!> HTML <!> |
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.
| Item | Example | Comment |
| IE | The string "IE" is a feature corresponding to the version of Internet Explorer used to view the Web page. | |
| value | 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 | 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 | 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 | Always evaluates to true. | |
| false | Always evaluates to false. |
The following table describes the operators that can be used to create conditional expressions.
| Item | Example | Comment |
| ! | The NOT operator. This is placed immediately in front of the feature, operator, or subexpression to reverse the Boolean meaning of the expression. | |
| lt | The less-than operator. Returns true if the first argument is less than the second argument. | |
| lte | The less-than or equal operator. Returns true if the first argument is less than or equal to the second argument. | |
| gt | The greater-than operator. Returns true if the first argument is greater than the second argument. | |
| gte | The greater-than or equal operator. Returns true if the first argument is greater than or equal to the second argument. | |
| Subexpression operators. Used in conjunction with boolean operators to create more complex expressions. | ||
| & | The AND operator. Returns true if all subexpressions evaluate to true | |
| | | The OR operator. Returns true if any of the subexpressions evaluates to true. |
Downlevel-hidden conditional comment
Below are two examples of a "downlevel hidden" conditional comment:or
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 or older than a specified version.
Downlevel-revealed conditional comment
Below is an example of a "downlevel revealed" conditional 'comment', which is not an HTML comment at all, despite the misleading name, using the default Microsoft syntax:This example shows content that should be exposed only to non-IE browsers, as the condition evaluates to "false" on IE, while the tags themselves are unrecognized 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:
While somewhat confusing in structure, this specific syntax is valid HTML and is useful for conditional sections intended specifically for non-IE browsers; if the condition evaluates to true, IE will then display the "-->" present before the HTML content. This problem is easily solved by prepending "" as follows:
This code displays on non-IE browsers and on IE 7 or higher.
The extra "false, everything within the conditional comment is ignored, and if true, the resulting tag
Conditional comments in JScript
Starting with Internet Explorer 4, there exists a similar proprietary mechanism for adding conditional comments within JScript, known as conditional compilation.Code examples:
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:
As a result, a possible way to detect Internet Explorer version using conditional compilation can be seen below:
However, conditional compilation is no longer supported in Internet Explorer 11 Standards mode.