2.1.2.4 X-UA-Compatibility Meta Tag and HTTP Response Header
Web developers can also specify a document mode by including instructions in a meta element or HTTP response header:
Webpages that include a meta element (see [HTML]) with an http-equivalent value of
X-UA-Compatible.Webpages that are served with an HTTP header named "X-UA-Compatible".
If both of these instructions are sent, the developer's preference (meta element) takes precedence over the web server setting (HTTP header).
For more information about how to control default rendering with document modes, see "META Tags and Locking in Future Compatibility" at [MSDN-METATagsLocking].
The X-UA-Compatible value determines Windows Internet Explorer’s document as follows:.
X-UA-Compatible value | Document modes |
IE=5 | Quirks mode |
IE=7 | IE7 mode |
IE=8 | IE8 mode |
IE=9 | IE9 mode |
IE=10 | IE10 mode |
IE=11 | IE11 mode |
IE=edge | The highest supported document mode of the browser |
IE=EmulateIE7 | IE7 mode (if a valid Quirks mode (otherwise) |
IE=EmulateIE8 | IE8 mode (if a valid Quirks mode (otherwise) |
IE=EmulateIE9 | IE9 mode (if a valid Quirks mode (otherwise) |
IE=EmulateIE10 | IE10 mode (if a valid Quirks mode (otherwise) |
IE=EmulateIE11 | IE11 mode (if a valid Quirks mode (otherwise) |
For example, in Windows Internet Explorer 8, IE=9, IE=Edge, and IE=EmulateIE9 result in IE8 mode.
Browser emulation modes are not document modes. They instruct Internet Explorer about how to select a document mode when a valid <!DOCTYPE> declaration is included.
The following diagram shows how Internet Explorer determines the appropriate document mode based on the meta element or HTTP header.