Conditional Comments

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 topics are discussed in this document.

 Terminology

 Benefits of Using Conditional Comments

 Syntax of Conditional Comments

o Downlevel-hidden Conditional Comments

o Downlevel-revealed Conditional Comments

 Version Vectors

 Examples

 Related Topics

Terminology

The following terms are used in this article.

Term Description

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.

Benefits of Using Conditional Comments

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

downlevel-hidden

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 [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