From 24 ways to impress your firends:

It is the destiny of one browser to serve as the nemesis of web developers everywhere. At the birth of the Web Standards movement, that role was played by Netscape Navigator 4; an outdated browser that refused to die. Its tenacious existence hampered the adoption of modern standards. Today that role is played by Internet Explorer 6.

………

JavaScript genius Dean Edwards wrote a script called IE7. This amazing piece of code uses JavaScript to make Internet Explorer 5 and 6 behave like a standards-compliant browser. Dean used JavaScript to bootstrap IE’s CSS support.

Because the script is specifically targeted at Internet Explorer, there’s no point in serving it up to other browsers. Conditional comments to the rescue:

<!--[if lt IE 7]>
<script src="http://ie7-js.googlecode.com/svn/version/2.0(beta3)/IE7.js" type="text/javascript"></script>
<![endif]-->

Standards-compliant browsers won’t fetch the script. Users of IE6, on the hand, will pay a kind of bad browser tax by having to download the JavaScript file.

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