Due to its relatively poor level of standards support, Internet Explorer tends to be the subject of most CSS hacks. Luckily, as of version 5, it deliberately supports a rather safe-to-use hack called “conditional comments”. Conditional comments are specially constructed HTML comments that Internet Explorer on Windows may treat differently from other browsers, optionally based on IE's version number. They can cause Internet Explorer to ignore the markup between comments or to include part of a comment as if it was regular markup. Conditional comments apply specifically to browsers using Internet Explorer's Trident layout engine, meaning IE-based browsers like Maxthon and Avant handle them like Internet Explorer does while browsers using other layout engines see them simply as regular comments. Internet Explorer on the Mac uses a different layout engine and doesn't support conditional comments.
The most beneficial aspect of conditional comments is that you are not relying on browser bugs when using them. When you use CSS hacks that rely on browser bugs, you run into the possibility of those bugs being fixed at an unwanted time or other browsers showing the same bugs. Conditional comments only work in browsers that specifically support them and claim to be based on Internet Explorer, which in this case all known browsers are honest about.
There are two forms of conditional comments: positive and negative. A positive conditional comment will expose the included markup only to web browsers that match the condition (meaning only the selected versions of Internet Explorer). A negative conditional comment will expose the markup only to web browsers that don't match the condition (meaning all non-IE web browsers and any versions of IE that the condition didn't match). Note that, since versions of IE older than IE 5 don't support conditional comments, you may get unexpected results in those browsers.
The syntax for conditional comments is as follows:
- Positive
- Negative
HTML
condition is one of the following:
IE- Any version of IE
lt IE version- Versions less than version
lte IE version- Versions less than or equal to version
IE version- Only version version
gte IE version- Versions greater than or equal to version
gt IE version- Versions greater than version
version is the version of Internet Explorer, typically 5, 5.5, 6, or 7
HTML is the HTML to be included if the condition does or doesn't match, depending on the type of conditional comment. When included, the HTML is placed right where the conditional comment is in the source.
For negative conditions, -->IE. The longer version is only needed when Internet Explorer might parse the contents.
The XHTML
To minimize the chance of your site breaking in future versions of Internet Explorer, read Preparing your site for IE.next.
