One of the most common operations performed in a Web page is to detect the browser type and version. Browser detection is performed to ensure that the content presented to the browser is compatible and renders correctly. The browser type can be detected using many different techniques. Most methods of browser detection make use of script on the server or client.
This article introduces conditional comments, which offer certain advantages over scripted browser detection techniques. Conditional comments make it easy for developers to take advantage of the enhanced features offered by Microsoft Internet Explorer 5 and later versions, while writing pages that downgrade gracefully in less-capable browsers or display correctly in browsers other than Windows Internet Explorer. Conditional comments are the preferred means of differentiating Cascading Style Sheets (CSS) rules intended for specific versions of Internet Explorer.
The following terms are used in this article.
|expression||A combination of operators, features, and/or values used to form a conditional statement.|
|downlevel browser||Any browser except Internet Explorer 5 and later versions. For the purposes of this article, downlevel refers specifically to any browser or browser version that does not support conditional comments.|
|uplevel browser||Internet Explorer 5 and later versions, which support conditional comments.|
|downlevel-hidden||A conditional comment block that is ignored by downlevel browsers. Internet Explorer 5 and later versions render the HTML content if the expression evaluates to true.|
|downlevel-revealed||A conditional comment block that is parsed by downlevel browsers. Internet Explorer 5 and later versions also render the HTML content if the expression evaluates to true.|
Conditional comments have certain advantages over scripting methods of browser detection.
- Low client-side impact.When a downlevel browser encounters a downlevel-hidden conditional comment, the browser skips over the HTML inside the comment, and the content elements are not parsed, downloaded, or rendered. This saves client machine resources.
- No script required.Conditional comments do not require scripting and DHTML, and when no scripting is used in a Web page, no scripting engine needs to be loaded. Conditional comments are processed during the downloading and parsing phase, so only the content that is targeted for the browser is actually downloaded. Conditional comments can be combined freely with other browser detection techniques.
- Separate code from detection logic.Using conditional comments, script logic can be separated into smaller and simpler segments of code, which are easier to maintain and understand. Plus, code segments are loaded only by the browser version for which they were intended.
- Cross-browser.Conditional comments have been around since Internet Explorer 5, but their use is not restricted to Internet Explorer alone. Conditional comments can be used to customize content delivered to browsers that support conditional comments and those that do not.
Syntax of Conditional Comments
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, as shown in the following table.
|IE||[if IE]||The only currently supported feature is the string “IE”, corresponding to Internet Explorer.|
|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.|
|!||[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.|
|true||[if true]||Always evaluates to true.|
|false||[if false]||Always evaluates to false.|
Downlevel-hidden Conditional Comments
The following sample shows a downlevel-hidden conditional comment, which contains a short paragraph of text
<!--[if IE 5]> <p>Welcome to Internet Explorer 5.</p> <![endif]-->
The downlevel-hidden conditional comment contains hyphens (“–“) in the opening and closing tag, similar to the basic HTML Comment. The condition appears in the opening portion of the tag, and [endif] is placed prior to the closing portion of the tag. The content is placed inside the comment tags.
Because the first four characters and the last three characters of the comment are identical to a basic HTML Comment element, downlevel browsers ignore the HTML content inside the comment block. Since content is effectively hidden from browsers that do not support conditional comments, this type of conditional comment is called downlevel-hidden.
If the result of the conditional expression is true, the content inside the comment block is parsed and rendered by Internet Explorer 5 and later versions. This behavior makes the downlevel-hidden conditional comment particularly useful for content that has been specifically designed for Internet Explorer.
The following sample illustrates how a client-side script block can be placed inside a conditional comment; in this case, a message is displayed in Internet Explorer 5 and later.
<!--[if gte IE 5]> alert("Congratulations! You are running Internet Explorer 5 or greater."); <P>Thank you for closing the message box.</P> <![endif]-->
In the preceding example, only the major digit of the browser version is compared because it is the only digit specified in the conditional expression. To compare both major and minor version numbers, specify both digits. For further explanation and examples on specifying the browser’s version number, see Version Vectors.
The downlevel-revealed conditional comment enables you to include content in browsers that do not recognize conditional comments. Although the conditional comment itself is ignored, the HTML content inside it is not. Internet Explorer 5 and later versions also parse and render the content if the conditional expression evaluates to true. The downlevel-revealed conditional comment complements the downlevel-hidden conditional comment.
The following snippet shows a typical downlevel-revealed conditional comment.
<![if lt IE 5]> <p>Please upgrade to Internet Explorer version 5.</p> <![endif]>
When comparing this type of comment to the basic HTML Comment, notice that there are no hyphens (“–“) immediately after the opening “<!” or immediately before the closing “>” of the comment block; therefore, the comment delimiters are treated as unrecognized HTML. Because the browser does not recognize the downlevel-revealed conditional comment, it does nothing with it.
Conditional expressions are often used to determine the version of the browser. The format of the version vector number must be defined correctly to obtain the desired result.
When testing the major browser version number, the version vector is an integer. To check for a minor browser version, follow the version vector by a decimal point and four digits. For example, the version vector for the release build of Internet Explorer 5.5 is 5.5000.
In the following example, only the major version number is specified; therefore, the sample evaluates as true for both Internet Explorer 5 and Internet Explorer 5.5.
<!--[if IE 5]> <p>Welcome to any incremental version of Internet Explorer 5!</p> <![endif]-->
The following test correctly identifies Internet Explorer 5.
<!--[if IE 5.0000]> <p>Welcome to Internet Explorer 5.0!</p> <![endif]-->
Here are some more examples of conditional comments.
<!--[if IE]><p>You are using Internet Explorer.</p><![endif]--> <![if !IE]><p>You are not using Internet Explorer.</p><![endif]> <!--[if IE 7]><p>Welcome to Internet Explorer 7!</p><![endif]--> <!--[if !(IE 7)]><p>You are not using version 7.</p><![endif]--> <!--[if gte IE 7]><p>You are using IE 7 or greater.</p><![endif]--> <!--[if (IE 5)]><p>You are using IE 5 (any version).</p><![endif]--> <!--[if (gte IE 5.5)&(lt IE 7)]><p>You are using IE 5.5 or IE 6.</p><![endif]--> <!--[if lt IE 5.5]><p>Please upgrade your version of Internet Explorer.</p><![endif]--> <!--[if true]>You are using an <em>uplevel</em> browser.<![endif]--> <![if false]>You are using a <em>downlevel</em> browser.<![endif]> <!--[if true]><![if IE 7]><p>This nested comment is displayed in IE 7.</p><![endif]><![endif]-->