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 following table indicates which document mode that Windows Internet Explorer 9 uses based on the X-UA-Compatible value.
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=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) |
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 Windows 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.
