https://msdn.microsoft.com/en-us/library/ff955275(v=vs.85).aspx
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 <!DOCTYPE> declaration is present)
Quirks Mode (otherwise)
IE=EmulateIE8
IE8 mode (if a valid <!DOCTYPE> declaration is present)
Quirks Mode (otherwise)
IE=EmulateIE9
IE9 mode (if a valid <!DOCTYPE> declaration is present)
Quirks Mode (otherwise)
IE=EmulateIE10
IE10 mode (if a valid <!DOCTYPE> declaration is present)
Quirks Mode (otherwise)
IE=EmulateIE11
IE11 mode (if a valid <!DOCTYPE> declaration is present)
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.
