Put simply, IE8 running in standard rendering mode will not display sites created to support IE7 very well, based on new standards that IE8 is supporting around HTML and CSS formats. There are two ways to resolve this. The first is to re-write your web application to render IE8 correctly or the second option ( the best to choose first off ) is to instruct IE8 to run a certain site in IE7 emulation mode.

On a Per-site basis, site owners and administrators can include the following custom HTTP header to force Internet Explorer 8 to render Web pages like Internet Explorer 7: 

X-UA-Compatible: IE=EmulateIE7

To add a custom HTTP response header at the Web site level in Internet Information Services 7 on a Windows Server 2008-based computer, follow these steps:

  • Click Start, click Administrative Tools, and then click Internet Information Services (IIS) Manager.
  • Under Connections, double-click the server that you want, and then double-click Sites.
  • Click the Web site where you want to add the custom HTTP response header.
  • Under Web site name Home, double-click HTTP Response Headers in the IIS section.
    Note In this step, Web site name is the name of the Web site.
  • Under Actions, click Add.
  • In the Name box, type X-UA-Compatible.
  • In the Value box, type IE=EmulateIE7.
  • Click OK.

To add a custom HTTP response header at the Web site level in Internet Information Services 6 and earlier versions, follow these steps:

  • Click Start, click Run, type inetmgr.exe, and then click OK.
  • Expand the server that you want, and then expand Web Sites.
  • Right-click the Web site that you want, and then click Properties.
  • Under Custom HTTP headers, click Add.
  • In the Custom header name box, type X-UA-Compatible.
  • In the Custom header value box, type IE=EmulateIE7.
  • Click OK two times.
Per-page basis
Site owners and administrators can include the following special HTML tag after the <Head> tag on the page: 

<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />

The following example shows use of this Internet Explorer 7 compatibility mode tag on a per-page basis: 

<html>
<head>
  <!-- Use IE7 mode -->
  <meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />
  <title>My Web Page</title>
</head>
<body>
  <p>Content goes here.</p>
</body>
</html>

 

For more information about the EmulateIE7 tag, visit the IEBlog Web site:

http://blogs.msdn.com/ie/archive/2008/06/10/introducing-ie-emulateie7.aspx