"Support" for IE11 whether on Windows 8.1 or Windows 7 appears to be limited to a Quirks document mode as the <!doctype> declaration is not supported. This forces IE11 to run in a IE5 compatibility mode. We have a requirement to move newer browser standards and not rely upon quirks mode. My understanding is document modes will not be supported in IE12. If newer standards are not supported in the near term our applications will no longer function and meet our business requirements.
Document modes and compatibility with Internet Explorer 11
Defining document compatibility (Internet Explorer)
Specifying legacy document modes (Internet Explorer)
2 major compatibility changes in IE11 you need to know about