As you all know, the Internet Explorer team has been working hard to make IE8 the most standards compliant browser around. Unfortunately, not all web sites confirm to these standards today. If you have concerns about your web site rendering correctly in IE8 standards mode, then there is some help for you out there:
First – you can address the issues on a page-by-page basis using the following HTTP meta-tag:
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />
This tag, when seen by IE8 will render the page in IE7 compatibility mode regardless of whether or not you’ve set IE to run in IE8 Standards mode or not. This is a great solution for that one page in the site that doesn’t quite render correctly in Standards mode.
Second – you can add this header via IIS to all of your pages either via the config files directly:
<?xml version="1.0" encoding="utf-8"?>
<add name="X-UA-Compatible" value="IE=EmulateIE7">
or using the IIS7 admin tool (which, as we all know, just updates the config files for you). For those of you running Apache instead of IIS, check this blog post on how to configure your Apache server to render pages in IE7-compatibility mode. You can also get more information about IE8 Compatibility Mode can be found on the IE8 section of the MSDN web site.
Source:→ Chris Koenig's Blog