Add http header to force IE8 to render in IE7 mode

In summary, IE7 compatibility support looks as follows:

IE=7

Display in IE7 Standards mode; Already supported in the IE8 Beta 1 release

IE=EmulateIE7

Display standards DOCTYPEs in IE7 Standards mode; Display quirks DOCTYPEs in Quirks mode; Available through the IE June Security Update for IE8 Beta 1

There are two ways to implement this tag:

* On a per-site basis, add a custom HTTP header 

X-UA-Compatible: IE=EmulateIE7

* On a per-page basis, add a special HTML tag to each document, right after the  tag

Implementing the HTTP header is beneficial if a site owner wants most of their site to render as it did in IE7 or if there are no plans to update site content. Inclusion of this header honors any Quirks mode pages that belong to the site.

Using the meta-tag on a per-page basis is beneficial when the publisher wants to opt-in specific pages to render as they did in IE7.

NOTE: The X-UA-Compatible tag and header override any existing DOCTYPE. Also, the mode specified by the page takes precedent over the HTTP header. For example, you could add the EmulateIE7 HTTP header to a site, and set specific pages to display in IE8 mode (by using the meta-tag with content=”IE8”).